If you don't care about the files on the computer reinstall windows, If not boot in safe mode, run all the scanners you can think of, run Ccleaner, reinstall Firefox, and you should be good :)
You will probably have to get a better CPU :( . 8 gigs of RAM is becoming the norm and is only around $20 more than 4 gigs. 800 MHz RAM is not very good, most people are getting 1600 mhz