I am having severe computer troubles and I really need help.

Symptoms:

My screen has pixels that are out of control in the sense that they spasm (not dead or stuck) However my monitor is in good condition. I pluged it into another computer and it worked like normal. I ran a program to search for dead pixels and that did not help. Besides the fact that whenever I am on the internet and I scroll up and down it is laggy, not smooth as you might see in older computers. When I boot up my comp this is what happens, The main 2 loading screens are visible but blocks of color changing pixels are ligned up orderly around my screen. When it gets to the login the names (of my family's logins) are blurry but I can click out of knowledge from past logins (where mine is), when I log in it is usually clear like normal for the first 2-3 minuites until pixels randomly cluster my screen.

Ok, so we know the screen is not the issue.

What I've Tried to do About It:

Next I check the graphics card (ATI Radeon 9800 Series 128mb) to make sure all connections were stable between monitor and card and motherboard. Besides this I installed the latest card drivers (ATI Catalyst Control) and did a Norton Virus, a Werbroot Spysweeper scan, ran CCleaner and tried a system restore. None of this helped.

The Day This Happened:

I was playing Counter-Strike Source like usual, had changed nothing in the game. Once I quit I read the Steam annoucement page to find to my interest that I could use a Steam beta version to incorporate my Steam community ID. I chose to do participate. Feeling tired I went to take a nap and watch TV for an hour or two to find my computer in screen saver as expected, once I wiggeled the mouse I found a red bar of dead pixels running vertically up and down my screen. I took it as a minor fluke and turned off my comp. The next time I turned it on there were 4 color changing bars running up and down my screen, from there it got worse, to the point were my entire screen was completely covered in color changing pixels. Once I changed a few things on my comp I got it so only the right 1/2 of my screen is pixelated. And thats wereI am now.

My Specs:

Manufacturer: Dell Computer Corporation

Processor: Intel(R) Pentium(R) 4 CPU 2.80GHz (2 CPUs)

Memory: 1022MB RAM (4x256mb DDR)

Hard Drive: 119 GB

Video Card: RADEON 9800 SERIES 128mb

Monitor: Plug and Play 19" Monitor - Dell 1800FP

Sound Card: SB Audigy Audio [CF00]

Speakers/Headphones: USB Altec Lansing Plug and Play Headset Mic

Keyboard: USB Root Hub

Mouse: USB Root Hub

Mouse Surface: A cool jell thing that prevents carple tunnel

Operating System: Windows XP Professional (5.1, Build 2600) Service Pack 2 (2600.xpsp_sp2_gdr.070227-2254)