Computer FPS Skippage, slowdowns

[fatboy2]

Pentium 4 CPU 3.20 GHz 3.21 GHz, 1.00 GB of RAM NVIDIA GeForce 7900 GT/GTO with updated drivers 945g series motherboad from MSI cable internet connection fresh format, newly installed windows when my computer was new i could put settings on decently high in games like CS:Source or WoW and my fps would be around 40-60, and it would stay constant and smooth. now its gets around 20-40 regularly and i get spikes, like either it freezes for a second or things slow down for a brief moment. sometimes its pretty smooth so i can deal with it but its annoying having my computer lag during a loading screen or when there is a lot of movement on my screen. i've tried lowering the settings but that doesnt help. anyways, any help or suggestions at all would be appreciated