Certain I have virus - but can't detect or delete

The other day I went on my computer to find my entire registry had been deleted and none of my programs would open properly. After fixing this problem, thinking that it may have just been corrupt entries or a one-off crash, I have noticed my computer doesnt work at all properly. Internet browser doesnt load any pages, and AVG cannot update, despite the laptop I am using to write this receiving wireless signal fine. I have also tried to begin scanning, but tried opening Spybot and AVG and neither of them open on the desktop (the processes are there in Start Manager, but I cannot access them. I have also tried this in Safe Mode, also to no avail. Can anyone please help suggest what is wrong, and how I can even just detect, let alone destroy and virus or malware thats infected my computer?
Many thanks.