One thing I used to do was to just delete the registry. That worked up to Windows XP, which was the last version of Windows that I used. The registry files were named user.d* and system.d* in your user directory and the Windows directory. I think it was just user* in C:\docume~1\user and C:\win*.
Cleaning the registry may not get you much more than just deleting it. Probably as good as doing a reinstall anyhow. Both leave you with a system that you have to reconfigure (although, a reinstall wouldn't remove the user.d* files), arguably you have to do more. However, if there's junk in your registry then you don't have much option if that's what is slowing the system down.
Once/if you have a clean slate, use norton ghost, or if you're up to the task, CloneZilla, to back your system up.
However, if you're just really pissed off with it all, like I was, try Linux or one of the unixes. There's some great sites out there that may help convert you. Things these days are a lot simpler than they were when I switched, somewhere around 1998, I've never looked back. Computers are generally not used like they were by most people in the golden days, a lot of the time most people don't have email clients configured and the system is just a facebook/gmail/hotmail client.
Switching is easy, you can try before you install, http://getgnulinux.org/switch_to_linux/. I describe on my site (link in sig below) why I think Linux is a better system. If you're not computer savvy, then it's even more important to not use something like windows that's so infiltrated with junk and spyware.
This is just my view, some will easily say, "Get a 'mac", in reality an Apple iMac is a unix system with a fancy GUI on top. Linux is a unix variant, its just very free and fun.