right click on mycomputer->properties->advanced->performace (the top option), click settings->Advanced (tab) theres a button almost to the bottom under virtual memory. its called change. click that ->
Increase the page file. you will have to select custom size instead of having the system automatically set the size..
To deermine the size, make sure its a good bit higher than the system set one, (what it is when you first see it) then decide how much space on your hard drive you want to give up..
Virtual Memory aka Page File (aka swap file in *nix systems) is basically a part of your hard drive that the OS can use as RAM, to save temporary data. Its reserved so the amount of space you set in the aformentioned panel will dissappear from the amount of free space on your hard drive
My dad has a 20Gb Hard drive, he uses office, a webmail client, and excel. so he doesnt need all 20Gb. He does however only have 128 Mb of ram. So in his case, I gave him 1.5 Gb (to start at) and 2.5Gb (maximum) of page file space. Its a balance of how much you used the drive.
Hope this cleared it all up for you.