I recently got a service call on a six year old PC, with Windows XP-Home on it, that had never had any service / maintenance done on it in its lifetime.
To say that the boot-up and everything else was slow, would be the Under-statement of the year.
Just clicking on a desktop icon, would give you time for a coffee break before you'd see anything new appear on the screen.

I booted up in SAFE MODE and went right to MSCONFIG and deleted everything in the Startup folder.
Then under Boot.ini I set the startup delay to 3 seconds, from the default 30 seconds. The PC had dual Processors but it was set to just ONE. I fixed that.

Then I went into Add/Remove programs and UN-Installed all the Spyware that had come with the PC, and everything else that the customer said he never used.

Then I ran Disk Cleanup in the Extended mode and took out a huge amount of junk files.
Then I ran my own XPCleanup.bat program and took out a lot more junk.

Finally after doing the cleanup, I booted back into normal mode and was able to start tweaking and tuning XP for optimum performance. As I worked, the customer was sitting right there, amazed at the improvements I was making.
Finally, when I was all done, removing viruses, trojans and spyware, the old PC booted up in about 45 seconds and shut down in 10 seconds, and when I'd click on something it would pop up immediately.

Cleaning up a DIRTY PC is not Rocket Science, but it does require some knowledge and some effort.

Cheers Mates!
The Doctor