There are so many factors that affect performance, that it's hard sometimes to know where to start. Without knowing a little more in the way of what you've checked, it'll be harder to narrow it down. For that purpose, I'll post a few questions and things to look at (if you haven't already)...
You said you have 1gb DDR, but what speed is it? PC2700 will be the minimum you'll want to run with the 2800+. Otherwise your FSB will be 266fsb instead of the 333fsb or 400fsb (depending on your chip) that the 2800+ was designed to run at.
Also, in system properties, is your CPU being identified correctly? and at what speed? And in Device Mangler, are there any devices displayed with a yellow exclaimation mark?
As dr_roberts49 mentioned, a spyware scan would be a great idea, as well as a virus scan... do you have any running processes that you don't recognize?
Please post back with what you find out