If your PC infected by a good and powerfull virus/trojan/malware, it may disable access to Task Manager, Regedit, MSConfig, etc and unable to run several native Windows programs to clean it up. So in addition to your favorits anti virus softwares, here are list of utilities that you should have

1. ProceXP a Task Manager replacement, no need to install just extract it and requires to run on Normal mode.
2. RegAlyzer a RegEdit replacement, requires to install, can be executed on SafeMode. By accessing to Registry you can enable again Task Manager, MSConfig, access to RegEdit and others windows option, tweak and settings by editing, adding and removing Registry items. (will not explained here). WARNING: Use it with extra caution. You do not want to broke you computer even more.
3. GS a Command Prompt replacement, no need to install just run it
4. Run Dialog Box Replacement, just incase the virus disable the Run dialog box.
5. Autoruns, a System Configuration utility (msconfig) alternative with many more options.
6. FreeCommander, a File Manager (Explorer) replacement. Usually virus disable options to view hidden/system files. Disable you to search infected files using Search menu. But this utility reveal them
7. PCMAV, PCMedia Antivirus is an excellent Indonesia Antivirus, just incase your PC infected by Local (Indonesian) virus which the ‘cure’ not yet available by all famous AV since they all products of foreigns.