Q:

How to set up a file server in a restricted corporate environment

I work in a big corporation, and the disk space my team gets in the corporate file server is so low, I am considering turning my work PC into a file server.
I ask this community for links to tutorials, software suggestions, and advice in general about how to set it up.
My machine is an Intel Core2Duo E7500 @ 3GHz, 3 GB of RAM, Running Windows XP Service Pack 3. Upgrading, formatting or installing another OS is out of the question. But I do have Administrator priviledges on the PC, and I can install programs (at least for now).
A lot of security software I don't even know about is and must remain installed. But I only need communication whithin the corporate network, which is not restricted.
People have usernames (logins) on the corporate network, and I need to use them to restrict access. Simply put, I have a list of logins of team members, and only people in the list should access the files.
I have about 150 GB of free disk space. I'm thinking of allocating 100 GB to the team's shared files.
I plan monthly backups on machines of co-workers, same configuration. But automation of backups is a nice, unnecessary feature: it's totally acceptable for me to manually copy the contents to a different machine once a month.
Uptime is important, as everyone would use these files in their daily work.
I have experience as a python and C programmer, but no experience whatsoever as a sysadmin, and almost nothing of my programming experience is network programming. I'm a complete beginner in this.
Thanks in advance for any help.
EDIT
I honestly appreciate all the warnings, I really do, but what I plan to make available is mostly stuff that now is solely on DVDs just for space reasons. 
It's 'daily work' to read them, but 'daily work write' files will remain on the corporate server. 
As for the importance of uptime, I think I overstated it: a few outages are OK, it's already an improvement over getting the DVDs. 
As for policy, my manager is kind of on my side, I will confirm that before making my move.
As for getting more space through the proper channels, well, that was Plan A, and it's still on the table... But I don't have much hope. I'm not as "core businees" as I'd like.

A:

I don't want to take an argumentative tone here, but I suspect your IT department isn't going to be too pleased with your plans. You're probably not going to get a lot of cooperation out of the Server Fault community to help you do this, either. What you want to do is, technically, very simple-- at least, on the surface. When you think about the longer-term ramifications and the other practical concerns that come into play, though, I think you'll find that your plans are actually ill-advised.
Here are some practical concerns re: your plans you should think about:

Availability (or lack thereof) - You say "uptime is important" and "everyone would use these files in their daily work". It's important that you have an understanding of what that means. Do you reboot the PC? Do you apply updates? Does the corporate IT department know that there will be others relying on this "server" and that unscheduled "outages" might hamper the ability of others to work? A desktop PC isn't a file server, and doesn't have the hardware features necessary to guarantee availability (RAID, power supply redundancy, ECC RAM, etc). You're risking the productivity of your co-workers with a "solution" that's not up to the task. This, alone, should be reason enough to scratch the idea. 
Legal compliance - The corporate file server computers, presumably, are audited for compliance with internal legal standards. Your ad hoc file server will not be, and you may violate document retention guidelines, etc. This could get your company into a lot of hot water and might reflect badly on you or your teammates.
Backup - You don't consider daily automated backup to be important, but IT probably does. 
So do the other people who would be doing their "daily work" on this machine, I'm sure. There are days when I wouldn't care, but other days when I'd be pretty unhappy if I lost even a day of my work. Monthly is a "crazy long" backup window if you're talking about the work of others, let alone yourself. You're also not planning for an off-site backup component, which means that all the data will be lost when the building burns, floods, etc. If you do opt to perform an ad hoc off-site backup yourself you could be risking exposure of corporate data in that off-site component.
Business continuity planning / DR - IT has, presumably, planned contingencies for business continuity planning and disaster recovery in the event that server computers fail, network infrastructure fails, etc. Your ad hoc file server won't be included in those plans and may present an impediment to business continuity when circumstances cause your machine to become unavailable to others (hard disk failure, OS failure, loss of network connectivity, etc). IT doesn't, by definition, know about your ad hoc file server computer. When they're planning for a "hot site" in a physical disaster they aren't planning to bring your ad hoc file server back up.
Migration concerns - When your PC gets replaced with another computer (with a different OS, a different computer name, etc) IT isn't going to be planning for an orderly migration of the "clients" of your ad hoc file server. When you get a new PC your co-workers may have lots and lots of "pain".
OS limitations - Windows XP Professional is limited to servicing 10 simultaneous file and print sharing client connections. Perhaps you don't need any more than this, but if you do this will present a very real, practical limitation to your plans.
Reliance on you - Once you leave the company what happens to your co-workers who are relying on your ad hoc file server for their work? You're not going to be there forever.

I'd advise you, as a professional sysadmin who has worked in "large corporate" environments before, to work through your management chain to get the resources you need requisitioned via the "proper channels" rather than taking it upon yourself to create something that will end up creating problems for you and the others who use it.
What you think of as a simple problem really isn't, and the reasons above (and more) are why there are "officially sanctioned" file servers meant for your team to use.

A:

My advice is - don't do it.  There is more that you're seemingly not taking into consideration aside from the (assumed) violation of IT policy: 

access time for your coworkers
synchronization - seems like you're creating multiple copies of files (whose data is where?)
lack of experience to be able to troubleshoot connection problems
integrity of backups
opening the door to malicious software

The biggest concern for me would be IT policy.  Is it worth your job to do this instead of requesting more disk space in a supported configuration?

A:

It's pretty simple:

Share the folder. Grant the Everyone group Full Control share permissions.
Access the security tab of the shared folder and add the domain users with the appropriate security (NTFS) permissions.

