I have a Windows XP system, and have recently upgraded by adding 2 1GB sticks of RAM to the 2x0.5GB already present. Since then, about once per day (the system is used 8+ hours per day), the system has suddenly and unexpectedly reset. On a couple of occasions, the system has frozen completely, only responding to the power button being held in for several seconds to force power off.

Nothing at all ever appears in the system event log that might indicate a possible cause - everything seems to suggest business as usual.

Sounds like faulty memory - but memtest86+ says otherwise. A full test, taking over an hour, found no issues.

The next likely suspicion, then, is that I've knocked something while installing the RAM. Trouble is, everything I can think of to test seems fine. I've opened up the case and prodded a few things around, hoping to get better contact on connections etc, but there's no sign yet as to whether that has made a difference or not.

I thought about a malware-related timing fluke, but again, so far as I can tell I'm all clear.

All I can think of to add to my checklist (mainly of things that I can't easily check) is...

• The power supply is (1) only 350W, (2) not necessarily the best quality, and (3) powering a Prescott P4 640 3.2GHz. Could that be borderline overloaded or about to die? How do I check?

• Is it possible that the CPU isn't getting cooled properly? I haven't had the fan past normal tickover even doing video encoding, and the only sane temperature reading from SpeedFan is pretty steady at 36 celcius, so probably not.

Any thoughts? Is there a standard procedure for diagnosing this kind of fault?