Another version of the MyDoom virus/trojan/worm is out, and apparently spreading like wildfire from the reports. W32.Mydoom.Q@mm Appears to come from a friend, having falsified the 'From' field. it will have an attachment of an EXE file, usually named 'photos_arc.exe'.
If you click on the attachment, it will infect your system and begin sending out infected mail using a built-in email server. It will also connect to another website and download/install a backdoor trojan on your system, which will then allow anyone with your computer IP address to access all of your files and remotely control your system, including planting more viruses on it or using it to attack other systems.
Remember, never click on ANY attachment that you aren't expecting! Even then, all EXE files should be regarded with extreme suspicion!