Unfortunately,we can’t tell you exactly.As matter of fact,1 year ago or so,a new “Trojan horse,info stealer” named carberp was created and was able to infect machines and evade almost all AVs’s detection,carberp is a clone of Zeus & Spyeye(you can google it) but even more sophisticated.The first detection of avast for that virus was Win32:Malob-BH.
Some examples:
http://www.virustotal.com/file-scan/report.html?id=d69efbd13dd8d3cbe9989841c5afb168df8eac819c84453f8301376ee35a4678-1277030353

It can also be something else,but just in case,keep a close(r) look at your banking accounts etc etc.