I hope I won't be fried on here for saying this, but CCleaner is a worthy tool for cleaning up the Windows registry of some junk that older programs leave in there. I've done MANY a clean and it never messed up my system (however let it backup the registry first!!). It also allows you to clean up the cache of different programs, which will also speed things up.
There will probably always be the occasional system that will have to be reformatted because it's really suffering from bit rot. But to make that installation go faster, slip-stream a Windows XP CD with the latest updates and store that installation on a USB key to make the installation a breeze.
Hope this helps.
T.J. Barber