It's important to have a firewall, as it keeps bad stuff from getting inside in the first place. (windows built in one doesn't count, get rid of that)
I like COMODDO because it prevents the registry from being modified, executables being executed by untrusted programs, and I can disconnect from the internet with a click of a button.
It is a little annoying the first week, telling it what programs are trustable, (like your web browser, steam, minecraft, ect ect ect) but after that it's pretty smooth sailing, at least it's better than a virus.
but yeah install what I listed and scan your PC with them and see what comes up.
To help with your virus issues:
1. Look at your task manager, and see if you have any processes running that shouldn't be. [hint Virus.exe won't be listed in bold red for you]
2. Get some anti-virus, anti-worm, anti-malware/addware, programs like I said above. (and run them!)
3. Uninstall unnecessary programs. (just go under programs and features to remove that old program your aunt put on their 5 years ago to open a .pdf file)
4. Defrag your drive. (try auslogic's disk derangementer, defragging doesn't have to do directly with viruses, but it can make your PC run faster.)
I would go more into detail on the "whys and how's" but I'm rather tired at this hour..
Also it's a good idea to not download the extra stuff provided by installers.
If everybody would take a 6 hour class on how to maintain a PC, the world would be better off...