You'll have to edit the file c:\boot.ini, it's a hidden text file, open it simply with notepad.
There will be a line like that:
Quote:[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP blablabla" /noexecute=optin /fastdetect
Add those two parameters at the end, like this:
Quote:[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP blablabla" /noexecute=optin /fastdetect /3GB /Userva=2560
Save your file and reboot the computer, that should do the trick.