If you have Avast...

[Fusion]

Currently Steam is showing up on their scanners as a trojan. Don't worry, you haven't been infected by anything. If you can't sign into Steam just disable Avast and don't go to any weird sites or look at any ads.

Edit: Avast released a hotfix, update virus definitions to 120205-2 and it should no longer see Steam as a threat. Also, you may want to try to remove the "infected" file out of the virus chest as the Steam Service update may not work otherwise.