Sluggish performance and freezing.

[Nevin]

can't figure out what's causing my computer to run so slow. Sometimes it will even freeze at the welcome screen when logging in.
Ive already checked everything off and starting up with a clean boot but the freezing will still sometimes happen.
Ive bought a new power supply in case the one my computer had was failing; the new one is much more powerful then the old stock psu to.'
With my copy of Windows Vista i did a complete reinstall of the operating system, wiping out any trace of information I had (keep in mind I backed up my pictures movies and games). But before I could transfer my backed up files It actually froze at the welcome screen when logging in for the first time like it did before the fresh install of vista.
I can't figure out what's happening here; my computer can perform more tricks other then freezing at the welcome screen, such as mega slow downs that render it useless for hours until it some how overcomes itself and runs fine again.

Computer is only 1 and a half years old and I keep it very clean inside the case and have made sure all the fans spin just fine and have even monitored the temperatures which all show up to be perfectly fine.

Windows vista Home Premium x32 SP2
E2180 cpu
3GB RAM
Made from HP