IE6 problem

I have a couple of W2K units that lately are constantly rebooting after getting on the net. The users can surf for a little bit, then randomly click on any link and the machine simly re-boots. If you don’t go on the Net, the units are working finr.Nothing has been recently added to these units.I scanned for viruses, trojans – clean. Hot fixes are up to date.
Any idea would be greatly appreciated!
Thanks,
Judy