
A hoax virus warning sent to a multitude of users can do almost the same damage as a mass mailing worm ;) You can always check whether a warning is true or hoax by going to
Symantec where they list all known virus threats.
Anything else you can type into Google.com and it will often tell you if it is a hoax :)
Dowload
AVG a very good free virus checker and keep it up to date daily and you will have very few problems