You can go out and buy a proper anti-virus such as Kaspersky, Nod 32 or Bit Defender. Or you could download Avast Anti-Virus, run and update and then run a boot time scan. This should get rid of most if not all of the viruses on there.
You shouldn't stick to just one program. Also do Spybot search & destroy, and ad-aware personal. If your computer has viruses, it definitely also has spyware.
and CCleaner
I have AVG. it's a wonderful program :)
but before you click restart or w.e make sure you update it first.
and i dunno once its like slow and been ruined by viruses i don't think theres any real way of getting it back to normal... only formatting would,
but it shouldn't get any slower once there are no more viruses/
Since you have AVG installed when you install Avast, click custom and untick everything cos otherwise it will clash with AVG
and once it done the boot up scan uninstall it otherwise it will start at start up.
and if you plan on using it afterwards, install spybot search and destroy, do an update and then do the browser immunization to help stop spyware getting into your PC, also run a scan.
Use CCleaner to get rid of any crap like temp files etc which can slow down your system.
But if you get an antivirus program which is a security suit like Norton internet security.. make sure you do not have spybot running the tea timer ./