suspected files is an incredibly infected machine.
I'm really most surprised that you've been able to use it at all with that level of infection.
I'm not sure about AVG recognising the quarantined files but as AVG and the other anti-malware programs you used report roughly the same number, then I suspect that AVG is checking the same files.
I suggest that you cancel out of AVG. If your computer is running well with those files from Spybot, Ad-Aware and CW Shredder now in quarantine, it is now time to delete those quarantined files. Get rid of them.
You haven't said what Windows OS you are using. If it is XP, disable System Restore, (Control Panel > System, System Restore tab and tick "Turn off System Restore").
Reboot your computer. (With XP, this will delete all the restore files where a lot of viruses and other malware can hide). Update the definitions for Spybot, Ad-Aware and CW Shredder, and run them all again. Quarantine any other infections. Reboot the computer again and check that it works ok, eg internet, programs, games, music, etc. If so, delete the quarantine items.
Make sure that the AVG definitions are uptodate, then run AVG and do a full system scan. Quarantine all infections.
Re-enable System Restore, reboot, and create a System Restore point immediately, (Start > All Programs > Accessories > System Tools > System Restore).
Download and install all Windows critical updates.
Make sure you have an effective firewall. Windows XP does have one, but it is not as good as third party ones like ZoneAlarm free.
I started with Spybot, Ad-Aware, CW Shredder. First time I ran them they pulled out over 10,000 suspect files. I quarantined them, not being certain that some of them may effect the operation or parts of the operation. It was suggested that I get a good anti virus so I downloaded AVG after reading mostly good things on different forums. I ran AVG and it identified over 10,000 suspect files. (Netsky was dominate). Now I'm wondering two things. First, could these files that AVG identified be the same ones that are quarantined? Secondly, it is taking an inordinate amount of time for AVG to "heal" the files. It is approx. 40% through the 10,000+ files (by it's own display) and it's taken over 15 hours. Is this normal?
Sorry for the long questions.