You could go download HighjackThis program
http://www.tomcoyote.org/hjt/
then run it and look for these lines
O1 - Hosts:
R1 - HKCU\Software\Microsoft\Internet Explorer\Main,Search Bar=
Also check these ones to see if they are infected
O2 - BHO:
O3 - Toolbar:
O4 - HKLM\..\Run:
O16 - DPF:
I had the same thing happen to me today and this got rid of it