I had the same problem. I tried the /3GB switch, but did not succeed.
I solved my problem with an older version of my NVidia GeForce 9600GT driver - I installed Version 6.14.11.7116 from Asus.com and everything works fine right now.
It seems, that NVidia has made some changes in the driver that did cause this problem. When I installed Win XP 6 month ago, everything worked fine. Until I installed the new driver from NVidia.
I hope, this will help others too.