also, its very common that you installed ram but it doesnt recognize it all, or your processor is overheating, etc.
You can see if its seeing all the ram in the bios
also some spyware isnt detected, see if there are any connections to your computer in the command prompt > netstat -n
also the RAM that you have, is it shared ram? if it is shared it is being used by sound card, video card (depending on the video card it can really take up alot of it), Disc drives, as well as obviously programs.
If your computer is not getting the required supply of power this can also cause malfunctions. Do you have alot of components in your computer? you mgiht need a bigger PSU (careful: dont get one too big or it will fry the box)
Are you constantly using pheripheral devices? this uses alot of ram.
Just some things to watch for, hope i helped
