Several companies make programs you can use to search your computer and locate or remove viruses from the computer. They can scan the existing files, or scan files as they are added - most do both.
These programs are only as good as their databases - which are usually up-to-date the day the program is installed (or the package is sealed), but which age. For this reason, most of these companies provide regular updates for free on their web pages. Read the instructions which come with your particular program, and follow them carefully.
Be aware that there is always a lag period during which your computer is vulnerable to any new virus. The period consists of
The time between when the virus is released, and when it is first noticed
The time between when it is noticed, and when detection and repair software is created
The time between when the software is created, and when you download it to your hard drive
You're only protected after the third stage. But it's better to be protected then, than not at all.