can you get the MOBO model number please?
From what I can gather from your conversations here, this is what you have...
AMD athlon 2200 with 266 FSB.
DFI Lanparty, unknown model.(possibly:LANPARTY KT400A or LANPARTY NF II )
One thing that I think of right away is heat. The processor and mobo that you are using, should have a thermal control system which in the case of a high temp reading, will clock your processor back.
Have you checked your heatsink and thermal compound? have you checked to make sure you have adaquate air flow through the case?
Your processor has been clocked back by 50%, which I would take as a safty measure the system is doing.
Take a look and see what you find. Remove the side, then look at the heatsink/fan assembly. is it working?