If you were to run a hijackthis log and check the O4 entries doubtless you would find an entry something like this....
O2 - BHO: Banner Rotator - {D117A61F-92C3-4450-A0C8-F425B14D4127} - C:\WINDOWS\system32\adrotate.dll (file missing)
Am I right? Good, check that entry and press Fix Checked...
HiJackThis:
==download hijackthis: http://216.180.233.162/~merijn/files/HijackThis.exe
-install it to a new folder alongside your program files and then rename the Hijackthis.exe to imabunny.exe.
-in that folder start HijackThis by dclicking the .exe; now close ALL other applications and any open windows including the explorer window containing HijackThis.
-select Scan Only, place checkmarks against the entry and then press Fix Checked.