mikey...
No, you do not need to delete those files. All AVG did was quarantine the dangerous files to nullify their harmfulness. They're fine where they're at.
As for Firefox, yes, it is a very good browser. It's best feature is it's endless customizability. A little like apps for iPhone users, add-ons for Firefox users are fun, interesting, helpful, powerful, bizarre, aesthetically pleasing -- whatever you want them to be. Essentially, FF can be
whatever you want it to be.
Beyond the customizability, it is generally faster than IE as well.
If you want a browser with less a little less polish and bells & whistles, but a lot of speed and still more potential, you could also consider Google Chrome. I know a number of people who really like it, too.
As for the best free program to remove spyware... that might not be as easy an answer as you'd think. IMO, there are several worthy options in this category. If you're looking for a one-stop-shop, do-it-all program, I'll recommend:
ccleaner,
Spybot Search & Destroy,
Ad-Aware (Anniversary Edition), or
Malwarebytes Anti-Malware.