Virtual memory is a disk file that acts as extra memory when the physical memory gets low.
To increase this on Win XP:
You must be an administrator-level account.
Right click my computer > go to properties.
Go to advanced tab, hit Settings in the Performance section.
Go to the advanced tab, press Change at the bottom.
Click custom size and set minimum to at least 50 MB, maximum to 2 times the size of your memory or more.
Press set, and hit OK.
You may be asked to reboot your computer. You have to do that to make it take effect.
As far as other windows OS'es, I can't remember how to do it.