[ez-1]

Samo and Sab, in most cases when your computer gets infected, the best and usually the only way is to format everything and start from the square one. What I mean here is that those bastards invites all their friends as soon as one gets through your firewall and virus scanner. Some of them know how to deal with different types of virus scanners and simply turn them off or tell them that there is n ovirus (some sort of Jedi trick I think).
Now we get to the solution part of the story. When you finally get your computer up and running again after re-installation of everything, you should install Symantec Backup Exec System Recovery (BESR). They have single user home edition out of that and it should be around £50. With that you need that external USB-hard drive just as FC said.
The beauty of BESR is that it takes backups automatically from your computer to that USB HDD and you can even have multiple versions out of your files. And it doesn't stop here. When it finally need to change computer, you can, if you wish to just clone the new computer from the backup of your old one and that shouldn't take more than 30 minutes (obviously depends how much porn you have on your old computer).