Need someone smart

I know something is broken, don't know what.

Internet connection only lasts for a minute or two. Then goes completely useless. No connection whatsoever.

Tried it with DSL, usb-connection DSL, usb-wireless cable connection, and ethernet NIC. (I have cable, gf has DSL)

I also did a clean format-install w/ windows 2000 pro & windows xp pro....same problems with both OS.

Not sure what is up. I assume no virus/worms/etc. since i just (today) did a clean install....not sure if this contributes but when i tried to go run-> cmd....the screen flickers the command prompt and then disappears....so can't do that function. (computer is an old dell 'P3')

If anyone knows what the hell this is (or what to replace), please help me....i don't even know who to ask/where to look....Thank you