That attachment is the MyDoom virus, if you've opened it (in Windows) then you are almost certainly infected. This is the Microsoft help page for removing it:
http://www.microsoft.com/security/antivirus/mydoom.asp
If you can't get to the Microsoft site it's likely that your hosts file has been used to redirect their site to somewhere else (probably the local loop 127.0.0.1) If this is so and you don't know how to edit hosts, ask away.
Generally never open an attachment unless you know the mailer *and are expecting a file from them*.
regards, Jerry