Win2K mystery reboot

Having a problem with a couple of my pc’s that I configured. I put a clean install of win2k, with all the updates along with standard office applications, office 2000, adobe 7.0 rdr, etc…
The users are getting a random reboot throughout the day sometimes a couple times without any warning. Virus scan comes up clean along with spyware scan. Did not tweak any settings pretty much a default install. The only common link between these pc’s are they have Citrix client v9.0 installed. Any ideas?