
The sensitivity is changed using the /S, /V, or /H parameter.  
The commands do the following:
/V - vertical sensitivity
/H - horizontal sensitivity
/S - both 

Follow the parameter with a number from 1 to 100 (1 - slowest, 100-fastest).
The default is 50.  You can type the command "mouse /S75" right from the
keyboard or add it to autoexec.bat.