A webcomics wife's blog that I read had a question about a noisy computer fan today, and since she's really nice, I wrote a description of how to open up your CPU and clean it out in case she didn't already know how. I thought I'd repost the comment here as a resource for anyone else. Others had previously commented on using a can of air to blow out any dust.
Ditto on the can of air. You'd be shocked how much gunk's in there. If you're doing it yourself and haven't before, opening up and shutting back up a computer's not much harder than filling your car's gas tank, probably on par with checking your tire pressure and oil level. It's perfectly safe so you 99.99% chance don't need to back anything up. If you've done this sorta thing before, or if you can figure it out on your own, ignore the rest of this comment. :)
Shut down the computer and unplug everything from the CPU (I usually start with the power cord, but I'm probably paranoid). Place it gently (don't drag or drop) on the floor in an area you don't mind getting dusty. If there's anything caked on the outside from animals or kids, now's a good time to get it - use alcohol based wipes. If you or any of the kids have asthma, consider wearing a dust mask before opening the CPU.
Inspect the back and sides of the case looking for a piece you can take off. There may be up to 4 screws holding on a side or top panel, they may need a screwdriver, wrench, or just your fingers; there may be some tabs you have to push while sliding a side of the case; you may have just one side come off, or it may be two sides and the top. Sorry I can't be more specific, they're all different. If you can't figure it out, take pictures from all sides (reassemble the computer) and post and we'll figure it out for you.
Once you're inside the guts of the computer, do NOT use water, and if you have to touch any of the circuit boards do so on the edges, gently. I take a can of air to the whole thing, getting out the big chunks first, then working methodically from the fans on down, getting every side of every item in there. Expect to use a half a can of air if you haven't been in there in a year. If there've been any other problems, you can also reseat and wires with plugs and cards that go into the motherboard. First time I did this I took a sharpie and put little notes to myself along the case when I figured out what certain parts of did, so when I had to change a hard drive later it was faster for me to find which thing it was.
Put CPU back together the same way you took it apart - things should only fit together one way, and don't force anything if you're uncertain. Plug things back in to the CPU - again, everything should fit in only one spot with the possible exceptions of phone modem wires (if it doesn't work one way, switch it), speaker wires (ditto), mouse/keyboard (look for colors or pictures), and USB wires (doesn't matter, your computer will figure it out). Again, I do power cord last b/c I'd rather safe than sorry.
Once you're done, turn it on and see what happens! Oh, and clean up the dust that's now everywhere. Good luck. :)