It sounds like you have a number of problems, but lets try and diagnose this.
What kind of video card do you have? An in-board video card is the worst, they use up a lot of GDI, and graphics heavy programs usually slow everything down. (a third party graphics card for any computer is a must, at least 32 megs)
Have you run all your virus programs and downloaded the latest signatures? It sounds like you might have a spiral or worm virus thats eating up your memory. A lot of viruses do this, and certain types of programs make them eat up more memory than normal. I remember the Back.Office virus did this to me a few years back.
Have you changed anything in the CMOS settings? (the settings you can access when your computer first boots up) You may want to go back to the factory settings. There may be a system conflict somewhere.
Download all the latest OS service packs, and your computer systems software upgrades. Windows has system upgrades for your OS, Dell has them for the system software and so do the rest of them. When you buy a computer they often install a tremendous amount of useless programs into your system tray.
If these are buggy they can eat up memory too.
Well thats all the diagnosis I can give at this point.
Good luck.