I have used an up-to-date version of Norton Antivirus and Lavasoft AdAware SE to check my system, but they aren't detecting anything abnormal. I've done ScanDisk and Defrags, but again, nothing apparently wrong.
It's a P3 with Windows 98, so it's not a new machine, however there doesn't appear to be anything wrong with the temperature or fans.
It seems to be a problem with the mouse - the more I use it, the quicker the system crashes. Could there be a connection?