Dust!

[Edgedamage]

Last night a co-worker brought in his pc for me to look at. He said it kept on locking up when he was doing video editing. Or anything that worked the processor hard. When I opened the case I found the processor heat sink was blocked by dust. Every single space between the fins was blocked. Well after cleaning out the heat sink the machine works great now. Just a warning check the inside of your case at least every four months.

[Minwah]

I've noticed the fans on CPU heatsinks (or any PC fan for that matter) are pretty good at collecting dust making it less efficient.

Can't say I check mine every 4 months (maybe every year   ) but certainly a good idea

[NoBonus]

You can reduce dust a little bit by buying or making filters for your air intake openings.  I made my own out of some velcro with a sticky back (you can find this anywhere, I think I got mine at Safeway.) and a thin green brillo pad.  Just stick the "hard" side of the velco (you know, the side that feels like a cat's tounge) around the air intake openings, cut the brillo to size and velcro it on (It should stick to the hard velcro pretty well).  Then every few months I either throw the brillo pad away and buy new or I wash it out.  It works pretty well.  I read how to do this out of a magazine or off a message board somewhere, so I can take no credit, but it does work well.

NoBonus

[Lance]

You can reduce dust a little bit by buying or making filters for your air intake openings.  

Using some panty hose over the fan works quite well too.  It will block ALOT of dust but still allow good air flow.

[Oldskool]

Air filters are great, but make sure they're CLEAN or they'll starve the air supply.

That would make matters much worse.....