Slow Computer

I have a computer that has suddenly slowed down considerably. It will take around 10 minutes to start up fully, and around 2-3 minutes to open any programs. Also when you type it takes a few seconds before the text appears on the screen. So thishas made the computer practically unusable. I have installed a new hard drive and tried to install a fresh operating system, however as soon as it gets to the non-dos section of the installation it slows right down and took several hours to complete. This definitely seems like a hardware issue, any ideas on what it could be?
Specs are PIII866mhz; 128MBRam 20GBHDD, Original OS Win98