Computer advice needed

We're having a serious issue with the computer (a four year old desktop running WinXP). It's randomly rebooting, a lot. And often after it has (or when we've started the computer up), it'll take it two or three goes to boot properly, restarting itself over and over.

We haven't installed any new hardware or software lately, and I can't find any viruses or malware (which could just mean I'm not looking correctly, of course).

Does anyone have suggestions on likely causes for this or things I should check?

Since the computer is getting on a bit, we're thinking of taking this as a reason to replace it. Are there any reasons to postpone buying a new computer for the next few months, or is now as good as time as any? I know there's always new stuff coming out with computers, but sometimes there is good reason to wait a bit. We're thinking do it now, before we get stuck buying Vista before the first major bug fixes are done ;)

similar genre

Hahahahahaha! I'm doing some Christmas shopping on JB HiFi's website, and ended up checking out the Atlantis s2 boxset (yes, already out in Australia, 99% certain it doesn't have extras, bah!)

Anyway, on item pages, it links to Other popular titles in this genre. So for SGA that seems to include: Angel, Stargate (fair enough), Futurama (heh), Dawson's Creek (...well I guess Joe was in it), Goodbye Mr Chips (have they seen something on SGA I haven't?), a Jamie Oliver dvd (I think McKay would be happier if there was more cooking in SGA) and Tipping the Velvet.

One is an American scifi action adventure show. The other is a period British drama about lesbians in the 1890s.

WTF? *g*