Unlike us old folks, computers don't really get weaker and slower just because they are old.
If the machine is getting "wonky", why not just replace the hard drive (keep the old one as a backup) and reinstall Win7 fresh. As long as you have the code, it should activate just fine. In my experience, the OS doesn't get wonky unless there has been a fair quantity of changes/updates/additions to the OS and programs, often leaving unused bits of junk in the registry file, pointing to places that don't exist any longer, and slowly bloating the system. An OS refresh should remove all of that, leaving a crisp and lean machine.
Also, if you've got an old spinner drive, replace it with a nice SSD. It will make things fly.