Re: Very Laggy PC!
You need to tell us what video card you have so we know if your memory is registering correctly.
That is to say that if you have a 64bit OS and a Graphics card of XXXMB memory, Windows must address that much in reserved space. This can make 4GB appear less than it is. However - I run a 768MB 8800GTX on a 32-bit Vista OS (don't ask) and my OS still shows 4.00GB RAM. Obviously this is incorrect. Windows 7 has fixed this issue (as well as Vista SP2 which I don't have - again don't ask) to my knowledge.
That said, run memtest ASAP, swap RAM, test it out.
And yes to concur with an earlier poster - heat absolutely can slow down a system as Intel chips for years have had speedstepping technology to protect the chips(as well as save power consumption).