You need to download and scan your system with Malwarebytes and SuperantiSpyware (links below)
If you are unable to download the software below then its likely you are being redirected or blocked from downloading security software by a trojan hidden in your device manager follow the instructions below and you should be able to download Malwarebytes and SuperantiSpyware which will remove your virus.
start > run > type: devmgmt.msc > enter > view > show hidden devices > non-plug and play drivers > in the list of drivers look for the following, right click and disable any that appear in your list:
TDSSserv.sys
TDSSxyz.sys where xyz are random characters
Clbdriver.sys,
Gaopdxserv.sys,
Seneka or Seneka.sys
Restart computer.
Download, install, update and scan your computer with both Malwarebytes and SuperantiSpyware, delete everything they find, if this fails to completely remove the virus/spyware download then update Avast and run a boot scan (click schedule boot-time scan and restart your PC), all the software below is free and safe to use.
Malwarebytes: http://www.malwarebytes.org/mbam.php
SuperantiSpyware: http://www.superantispyware.com/download.html
Avast Anti-Virus: http://www.avast.com/eng/avast_4_home.html
To protect your PC in the future you'll need to install anti-virus software and a firewall, id recommend using Avast anti-virus and Comodo firewall both are very effective and completely free, you should also switch to Firefox 3 if you are browsing with Internet Explorer as its much safer and a lot easier to use. (links on my profile)
Remember never download anything thats recommend by a pop-up.