Heating problem: It's actually BSOD'ed more often at night than day. Also, my flat has been a lot hotter than this during the summer and never had any problems. For three years running.
Dust: I cleaned out the dust in my PC a month ago.
Download: It was only 10-15 kpsI've downloaded 100kps and had winamp playing fine.
Reboots: I've had my PC running for seven weeks without rebooting and had no problems - obviously, these BSOD's mean that I've never had it up for long. But I'll say again that over the past five or so days, this has been a very sudden thing, as I've not had BSOD's for probably well over a year. For this to be heating, rebooting, etc... you'd think this would have happened more often in the past three years.
Defrag might be an idea, although I do have over eight gig free on a 40 gig hard drive.