A virus is capable of replicating itself to every file on your computer as well as system files that the computer needs to run properly. If a virus infects systems files, the computer must be formatted and the operating systems must be re-installed. This would cause the machine to lose all of its saved files. Viruses can be spread by several different ways. If the virus software being used is current enough, it should be able to destroy the virus.
Viruses can lay dormant on a computer for some time.
If you suspect you have a virus, run a virus scan of the entire computer. There are several different ways to prevent your computer from contracting a virus. Do Not open any attachment unless you are sure of the origin. Do Not open any email if you are not sure of the sender. Do Not open any file from a floppy disk unless you are of the contents and origin.