Leo says that dogs age 7 years for every human year, and with computers it's even worse. Computers probably age 15 years for every 1 year. So it's not surprising that it's slow. If he were to restore it to the way it came from the factory, it would be exactly as fast as it was then. The problem is, over time, we want to use newer software. The web has also gotten a lot more elaborate. If he wants to do exactly what he was doing in 2005, it would be as fast as it was then. But that's not the case, he's doing more modern things and using more modern software.
If he has the system restore disks, they're easy to run. He'd just put the disk in, restart the computer and it will ask if he wants to wipe the drive and install Windows. Of course, he should make sure to back up first. The first thing he should do after installing Windows is run Windows Updates. If he doesn't do this, he won't be secure.
If he doesn't have his recovery disks, he could contact HP and they could send them for a nominal fee. He could also put Linux on it, such as xubuntu which is designed for older computers. It's actually more secure than Windows too, and kept up to date.
The good news is Windows 8 is coming out soon, and there are lots of new Windows 8 machines available. He'd definitely benefit from a new computer at this point.