The most harmless ones are just that - a chain email just to see how far it gets spread around the world
Some of them contain instructions to carry out certain actions like delete the 'teddy bear' icon called Jdbgmgr.exe or warning you about the WTC Survivor virus.
NEVER follow any instructions to delete things or change settings on your computer.
If you want to know whether it is a hoax go to Symantec's website on Click Here
These hoaxes prey on peoples' willingness to help their friends and contacts and at best just jam up the internet with unnecessary emails and and worst create problems with instructions or even carry viruses!