Hairdryer = hot
If you're experienced with PCs: Get hold of a blower (someone you know must have one), a toothbrush and take your entire pc to bits.
Take out fans, heatsinks, hard drives, ram and optical drives. Lay all the bits out on a static-free flat surface and make sure you're not going to nuke stuff with static.
Individually clean each component with the blower and then the toothbrush, making sure to get dust out from the slots in heatsinks and corners of your case. If your case has dust filters, clean 'em the same way (carefully). You can wash them if you want but make damned sure they're dry again.
Re-assemble and go.
If you're not experienced: Form O shape with mouth, locate dusty bit and blow. Alternatively, pay some dorra to a PC place to clean it for you.
Seriously though if you're comfortable with taking your PC apart then it's actually quite therapeutic to take stuff apart and make sure it's good and clean.