You might have better results in adding more RAM. If you don't have the following amount you'll see a slow down:
XP or Windows 7: 2GB
What exactly is your problem. Do things just feel sluggish?
I'd also recommend an SSD, but it's really not worth spending money on this system. Even if you could overclock that wouldn't make much difference.
Your main issue likely isn't the CPU even though that is an issue. Your main issues are likely:
1) insufficient RAM
2) software issues (unless you've reinstalled XP in the last two years)
3) slow hard drive
My Aunt's Pentium 4 system (single core) only had 256MB of RAM in it. I put in in 2GB (2x1GB) instead for $30 at the time and the difference was incredible.