I had 8 Dell Optiplex 760 running Windows XP with Symantec EP in a network domain environment, that would freeze up randomly. I read through several forums and threads. The only thing the stopped the freezing was updating the drivers to the latest version. Go to the dell support website (www.dell.com/.../optiplex-760) and download and install the following updates (verify the release date to make sure you are downloading the latest driver):
BIOS (latest version A15)
AMT SOL/ LMS CHIPSET
This has permanently fixed the freezing issue for me. You may also want to check device manager to verify that you are not missing any drivers.