run <b>msconfig </b>from the run menu. Check for startup applications. Disable them and see if the problem goes away. Are they running on active directory? Boot them locally.
Are they all imaged machines? Same installed applications? Try running a different profile or removing applications they have in common.
also check your Power management to see if something might be conflicting. after checking the above steps and this and still no luck I would run a spyware check. if they are imaged machines did you run newsid to randomly create a new SID for each machine? if not then the network is seeing them all as the same machine even though you change the name of the machine. this can cause a lot of problems.