Help regarding overheating on a Dell Dimension 5100C

[novemberward]

Hello,
I bought a Dell Dimension 5100C about 8 months ago and have had no troubles until lately when the computer has taken to shutting down at random, causing the power light to blink amber (indicative of a problem with an installed device, according to the manual). Otherwise it runs fine, a little slower than it used to, but something is definitely wrong: the four diagnostic lights are not on at all- even though the computer is on and seems to be working fine at the moment. The little battery-shaped light below them flickers occasionally, not in a repeating pattern though. I opened the case and observed that the power supply and fan, the heat sink and blower assembly, and the hard drive all feel especially hot.
My question is how to check the heat sink for dust and how to check the power supply for overheating problems. There's a small vent/fan right on the underside of the power supply and it's constantly blowing hot air. Same with the heat sink. Any help on how to fix or check these things would be greatly appreciated- I don't know how to open them up to see if they are indeed full of dust. Thanks-

[pam123]

Oh yeah, those Dell cases.
Where is your manual? The "How to" is there.
While your hunting for it go buy a can or two if compressed air.