Multiple Computers Crashing but no BSOD

Hello,

I look after about 30 PC's for a small company, as of three days ago 5 of the machines have started locking up with the only way to bring them back to life is cycle the power. They are all HP machines of differing ages and specifications, they are all running windows 7 pro x64 and are patched up to date. If I look in event viewer their is nothing to indicate what the problem is. There doesn't seem to be pattern to the conditions where they freeze up, they can be not logged on to, or being used.

Normally if it was one PC I would assume its a faulty PSU or a dodgy stick of RAM but seeing as its effecting 4 PC's I doubt that is the cause, if I run a livecd of linux they do not crash at all (BitDefender Rescue CD), this points to the OS being the cause. It's driving the users up the wall and I cannot seem to trace the problem. Monthly security patches were applied the day after patch Tuesday and the problem didn't surface till over a week later so I doubt they are the issue.

FYI, they are all part of a Domain, and run Kaspersky Endpoint Security 8

Anyone got any suggestions as to what the problem might be?

Yard Dogs fix is the one that has worked, never knew you could run the upgrade option with the same version of windows as I was already running!

Thanks chaps!