...and if it IS dirty inside, use a can of compressed air (along with the alcohol and Q-tips) to clean it out. Do NOT use a household vacuum cleaner like one person I read about. Those vacuums generate a lot of static....he zapped his components real good. And do NOT use a garage-type air compressor like ANOTHER guy I read about. After he did that they were picking computer components out of the trees in the next county over.
Proper way to use the compressed air can: short bursts, not long drawn out blasts... otherwise you'll ice up the can too quickly. You can get those at any electronics store like Best Buy...even Walmart carries them. The 90% alcohol you can find in any drug or grocery store...just make sure it does not have any gels, aloe vera or fragrances added to it. If you can find 100% isopropyl, that's even better.

As for freezing up...like the others mentioned: could be anything from an overheated vid card to just angering the gods in Olympus. Steve had the first clue...perhaps a program conflict (something you installed) or a virus you downloaded. I'd reboot and run an antivirus program, then try a system restore as Spyware Dr suggested.

(Don't you all love it when I make suggestions that have already been made or clearly implied? How original of me....)