I've never had a Windows pc get slow after 6 months... Unless I've beat the snot out of it as I just don't care. But I'm an Admin, user boxes don't usually have such an issue. I have a 10 year old Windows 7 box that's as fast as it was 10 years ago.

But... If you install/uninstall a lot of stuff, over time that can cause issues (because Uninstallers are notoriously lazily compiled - I say this as an app packager of 20+ years.)

I used to say Windows Reg cleaners are snake oil, but on some systems it can really help with the uninstall issue - lots of crap, especially stuff related to context menus, can really slow it down. The only one I've ever recommended is Crap Cleaner - I've seen it revive a test machine that had gotten slow from a billion installs/uninstalls, testing lots of iffy software, etc.