MAJOR CoD4 problem!

Hi there guys! I have a major problem with running CoD4. I had this problem for a while now, for about 6 months, but it kept disappearing and coming back. Now it stayed for 3 days which is unusual.

Back to the problem:

Some objects, like boxes, guns, grenades, pillars, really any kind of object can become black on my screen, sometimes I can't even snipe because when I try to aim my whole screen is black, and it lasts for a whole war.

Here is the link of what it looks like:
http://www.xraygaming.com/index.ph [...] d=specific
(Its not black because I tried to alt-tab)
On the left there should be the Sniper image, as he sniped my clan mate, but as you see its blackened. So for the whole game I see the sniper black. I could live with that but the game keeps lagging and dropping to 20 fps, sometimes it says connection interrupted and I'm stuck for a minute or so.

You can see my OS on the screenshot.

My PC configuration is:

AMD Semprom 2800+ 1.61 ghz (way too weak for CoD4, but that shouldn't cause Graphical problems, right) - 3 years old
ATI Radeon HD3850 512 MB DDR3 - half year old, with latest drivers.
Nvidia NF4X - motherboard, also 3 years old
1.5 GB of DDR1 400MHZ RAM, 2x256MB 3 years old, 1 GB 1-2 years old.
HD 120GB SATA - methinks!

Could use some help immediately as I play many EU ladders in CoD4 and my team can barely count on me right now.

I know, but I need a solution that will hopefully last until the end of the summer, then I prob get my new one. HD4890 x E8400 @ 4ghz, 6GB RAM etc!