Downloading a 'small' program - a video editor, document editor, screensaver slideshow app, etc., is a risky gig in Windows. A 'bigger' program, like a browser, is a walk with death aka, BSOD.

FireFox is a good idea for a FRESH install of Windows, IF IF IF downloaded from Mozilla.org .

You might be able to clean up Windows with anti-virus, malware removal programs, but no telling WHAT THEY'LL LEAVE BEHIND (and they DO leave stuff behind).

Depending on what scan programs you run to do some cleanup, and more importantly, what they find, you might want to re-install W7. AND setup dual boot with Linux, for comparison when Windows starts showing problems with speed, etc.

Meantime, pull up Task Manager, and see under 'Performance', if your CPU is getting bogged down with non-Windows Processes.....