Hello, you have downloaded a trojan, possibly from a spam email. Symantec have developed a removal tool for this particular virus. Trojan.Vundo Removal Tool - Symantec.com
The download link for the removal tool is at the top of the page, follow the removal instructions at the top of the page.
Be careful as this trojan seems to spread itself through networks so if you are connected to any network, disconnect immediately to try and stop the virus spreading.
If you need any other help, or have any other problems removing the virus then get back to us and we'll give you all the help we can.