1 (Worst)
If, despite all the security measures you take, your computer gets infected by a virus or worm, it is necessary to disinfect your system somehow. While for some viruses the only 100% realiable method of disinfection is restoring your system from backups, for many common infections this is not really necessary and the virus/worm can be removed quite easily.
Removing the infection
The worms often schedule themselves to be run automatically when you start your operating system; some of them even register themselves to be run when any other application is started. Removing such a worm is not as simple as deleting it - when you just delete the worm file, your operating system might not be able to start your applications (such as Explorer) any more.