Do you see a Blue Screen of Death (BSOD) just before the machine reboots, as Karen had in her description? If so, in what module does it indicate the error resides? That would be a big hint as to what the cause is. You may also find something in the Event Log, which is found under Administrative Tools. That could potentially tell you where the problem is.
I've found that often a manufacturer of a peripheral will issue a buggy driver, forcing either an upgrade or a downgrade of the driver to one where the bug doesn't exist. Because the problem driver often is in the latest release, it's often good to go the the peripheral's manufacturer's website to grab the previous driver version. Plus, what's available on MS Update often isn't the correct driver that the manufacturer uses. The MS Update driver is often a generic driver written for the peripheral's principle chipset and is not customized for the actual hardware.
For instance, nVidia (or ATI, etc.) makes the basic chipset for a large number of video cards. Each video card manufacturer licenses these chipsets from nVidia, but customizes the card for their own purposes with completely different drivers and included software. I prefer to trust a manufacturer's website to MS Update.
