Have you looked in your system's Event Logs for an indication of what type of errors you are getting?
Another thing to check is your CPU and case temperatures.
~Edit~ Download SpeedFan
to check your temps.
SpeedFan is a freeware program that monitors voltages, fan speeds and temperatures in computers with hardware monitor chips. It can even access S.M.A.R.T. info for those hard disks that support this feature.
Edited by TMacK, 02 August 2007 - 11:25 PM.