Can i still install norton anti virus on windows 7?

I've had my laptop a little over 3 years and it's been slowing down over the years. For example, it takes a couple minutes to open Mozilla Firefox. I know it has to do with all of the junk on my hard drive, but I don't know how to go about cleaning it out. Should I completely wipe out my computer and start anew? If so, - will I have to re-order/ pay for programs such as Norton Antivirus? - will I need to install another copy of Microsoft Office/ buy a new version? Or, is there any FREE program that can help clean out the junk on my hard drive without completely wiping it out? If so, - Please name and describe the program. Sorry for the long question, but thanks in advance for the help!  

improve concentration