Why is my PC running slow?

I have a DELL Dimensions 3000 running on Windows XP professional.
3GB RAM
250Gb HDD + Additional 500GB

The machine runs like a dog, so slow. I thought the RAM would improve things.

In Task manager the CPU usage is around 100% most of the time and the PF can be up to 1.5GB in size.

There are times when it is so slow that changing from one screen to another can take 45 seconds and it is like a blind being drawn down to reveal the next page.

What can I do to improve performance/speed apart from buying a new PC?