If you have the installation disc, you can run "SFC /SCANNOW" with disc inserted. It will look for and replace/add damaged/missing files. Also you can check Services in msconfig, hide all Microsoft services and sift through the remains. Then you can run services.msc and look for services belonging to a particular software, stop and disable when found. Run CCleaner and uninstall the malicious software, add the Documents and Settings\<user name>\Local Settings\Temp, Temp Internet Files, History folders to cleanup.
I do a lot of gaming and I replaced my Avast with Comodo. AVG is easily disabled by porn websites.
Otherwise, you can run a secondary install of windows, to keep or backup files and data, then reformat and reinstall windows if all else failed.