I would have to say backup important files, and wipe clean. Unless its adware running in the background (usually Ad-aware or MS Antispyware beta will take care of that) it is prob. something hosed inside the registry. It could also be a memory issue or a hard drive space issue? It also could be fragmented files on the hard drive. (That would explain "slowness" on the install) With that in mind, no defrag in the world is going to fix it if it’s that noticeable of a problem. As stated before, your best bet is to backup, wipe clean, reinstall.
On a personal note, I do this every 6 months or so, just because of the speed. After about 6 months, it gets "sluggish".
Thank you for the advice. I just might have to do that.
I've got to run a quick errand. So I can't respond for about half an hour.