+1 to everything mentioned. You might try this...download a program called Malwarebytes, it's free, and Free Registry Repair from regsofts.com, it's free as well. Install and run the Malwarebytes first, the follow it with the free registry repair. The first time you run Malwarebytes, run the full scan, the quick scan will suffice from there on out. Be prepared, it takes quite a while the first time and you shouldn't do anything on the computer while it's running. Then run the registry repair, fix all the listed problems. Run it 2-3 times to get as much as possible. One note, there are some windows processes that create registry links and then abandons them when the process completes, so you can never get to zero in the problem list. 40-60 is a decent place to stop. Also, as of yet, I have never had a crash caused from "fixing" a listed problem, but it gives you the option to create a restore point before fixing problems.
There are other programs out there that do a decent job as well. I've used these two for about 3 years and have had really good results. The two programs are legit. You can purchase Malewarebytes to get realtime protection, the free version has to be run manually. Free Registry Repair is outright freeware.