
I had this problem some time ago.  Some BIOSes do not automatically install 
COM3 or COM4 in the port tables. Programs like most modem programs which 
write directly to the port work fine but anything that uses a BIOS call 
fails. Find a BBS or FTP site where you can get a copy of PORT FINDER. Put 
"device=pf.sys" in your config.sys or run pf.com from your autoexec.bat. 
This little program will locate all existing ports and make sure the BIOS 
tables are updated. It works great. PF will also let you swap ports and 
such also if that is of any value to you.