See if you have RAM limit set in Win XP. To see it try this :
Start-> Run -> type "msconfig" without the quotes
go to BOOT.INI tab and see if you have something like this :
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect /maxmem=256
If you have, click on the "Advanced Options..." and uncheck /MAXMEM box.
OR
Edit "Boot.ini" on system drive root and delete the red part.
Beware of he who would deny you access to information, for in his heart he dreams himself your master.
Commisssioner Pravin Lal
"U.N. Declaration of Rights"