John was smart enough to send me a detailed listing of his hardware loadout, and I quickly spotted a possible culprit: mismatched RAM. His system has a pair of 512MB RAM modules and another pair of 1GB modules.

In theory, this should work fine--but in reality, it can cause problems. I suspect that the system had the two 512MB modules when John first bought it, and that he added the extra RAM to improve performance. Smart move, but if the newly introduced modules aren't exactly the same as the originals--same speed, same transfer rate, etc.--they could definitely cause random reboots.

My advice, John, would be to pull the two 512MB modules and see how the system fares. (Vista can get by just fine on 2GB.) If it still reboots without warning, try going back to just the 512MB modules. (It may not be a RAM mismatch that's causing the problem, but rather a bad RAM module.)

Keep in mind, this is just one possibility. Random reboots can also be caused by bad motherboards, glitchy video-card drivers, and other hard-to-troubleshoot items. To help narrow down the problem, check out Scott Dunn's .

If you've encountered a problem like this one, hit the comments and let your fellow readers know what caused it--and how you resolved it.