I'm happy with Avast antivirus and Superantispyware so I'm going to stick with those. As I understand it Microsoft security essentials will clash with other ant virus programmes and they don't recommend running two at the same time.
demoivre - I also use Avast and grabbed Superantispyware yesterday. I also have Malwarebytes and Windows Defender (which is part of MSE). I highly recommend Malwarebytes and yes if I did opt for Microsoft Security Essentials I would de-install Avast. From what I read yes MSE is good but Avast for example or Avira is better.
The excellent Gizmo Richards site recommends MSE as the best of the free anti-virus programs for users who are not too technically minded. [broken link removed]