Probaly is tired and knackered. Could do wiv a rebuild.
I looked at a friends PC the other week and it was cheaper to buy a new one rather than go for any upgrades (the upgrades were very limited anyway cos of the age of the motherboard ie max 1Gb of RAM).
If it was me and I didn't need the all-singing hi-power games/work-station box I would go for a cheap PC (software is the expensive part).
I would even consider buying some of the stuff these people recycle (I have dealt with them before for power supply units) -
http://www.computer-reclaim.co.uk/Cheap and cheerful, almost throwaway, but would do basic Internet and word-processing stuff etc.
And yes, I know that people might say it's a a gamble and that I would go for it cos of my background, but I had a friend who used to buy an old banger every year and bin it when the MOT ran out, similar sort of thing really. Did what was asked of it, no more, no less.