My children have been playing games on my computer and have done something to it. I took it in to a PCdoctor place and they cleaned it and sold me a new power supply. Of course that has only helped the fan not to run at high all the time; it still has many problems. Slow startup, choppy/stuttering audio/video (took 4 hrs to watch a 2 hr movie), crashes, can't run multiple programs concurrently, Word and Excel take at least 2 min to start, to name a few. I've run a virus scan from TrendMicro which found nothing. I guess they could have downloaded something that has gummed up a driver. It's almost as though there is something running in the background sucking up the CPU. Aughhh!

Someone please help . . .

Run an anti-virus / anti-spyware scan to de-clutter your computer of any nasties.
Run a disc defragmentation to slightly speed up your computer, this will take between 4 -> 8 hours to complete and don't run any other programs while it is working.
Also install the latest drivers for the Graphics card, if you have one.
Also what Audio/Media player do you use?
If it isn't Windows Media Player 11 or iTunes/Quick Time player, then i would consider getting WMP 11.
Also, how much RAM do you have?
This can be found by:
Control Panel -> System -> and it is shown on the bottom left of the smaller window.