|If I remember correctly|
right click on My Computer, choose Properties (you can also go in through Control Panel, System as an alternative), select advanced tab, virtual memory. See how much you have showing. you should have at least 500mb but I like 1000mb and you can if you like add another equal amount on the second drive if room is not a problem (not needed though) save and close.
(I say if I remember exactly since I am now running Windows 7 64bit but my prior main machine was XP Pro for almost 9 years, I just cant do a 'practice run for you to remind myself)
The old rule of thumb was equal to your RAM but that was when RAM was typically 128 or 256 (or if you had money, 512) back in the Windows 95 days.