You have an odd setup for RAM if you are running two sticks for a total of 3gb, and that will if anything casue your PC to run slower than it could be.
Is your PC shutting down at any time or when you are gaming or websurfing, what are you normally doing?
How have you tested the PSU and RAM?
And what is the model and wattage of your PSU?
You can DL HDTune and run a quick test to check the health of your HDD's, it wouldnt hurt to try and let us know what you find out.