I would say no to the clean PC web sites. The best thing to do is upgrade. Memory and disk space is cheap. You statement "Over the years" tells me your hardware is old. find someone to build a PC for you - store bought ones have too much garbage loaded on them. If you can't afford that, then the best thing to do is save off your data files and pictures, and have someone wipe the drive and reinstall the OS.
I would also say no to open sopurce of free AV programs like AVAST. I use Trend Micro. Subscription is inexpensive.
Or, you can look into tryin to clean things up yourself, take a look
at this web site.