[Xrati]

Do the Control-Alt-Delete and see how many processes you are currently running. This will give you a good idea of how much of your resources are being devoured by 'background' programs. Virus programs as Norton or McAfee will totally suck up your resource space.
If you have more then 40 processes running (ideally you want between 32 to 40) you will need a program (like Spybot S&D or there are others) to turn off some of those processes, reclaiming memory for you to run the programs you want.