Unexplained computer behavior may be caused by deceptive software
http://support.microsoft.com/kb/827315

Run a /thorough/ check for hijackware, including posting your hijackthis log
to an appropriate forum.

Checking for/Help with Hijackware
http://aumha.org/a/parasite.htm
  http://aumha.org/a/quickfix.htm
  http://aumha.net/viewtopic.php?t=5878
http://wiki.castlecops.com/Malware_Removal_and_Prevention:_Introduction
http://mvps.org/winhelp2002/unwanted.htm
http://inetexplorer.mvps.org/data/prevention.htm
  http://inetexplorer.mvps.org/tshoot.html
http://www.mvps.org/sramesh2k/Malware_Defence.htm
http://defendingyourmachine2.blogspot.com/
http://www.elephantboycomputers.com/page2.html#Removing_Malware

When all else fails, HijackThis v2.0.2
(http://aumha.org/downloads/hijackthis.exe) is the preferred tool to use (in
conjuction with some other utilities).  HijackThis will NOT fix anything on
its own, but it will help you to both identify and remove any
hijackware/spyware with assistance from an expert.  **Post your log to
http://aumha.net/viewforum.php?f=30,
http://forums.spybot.info/forumdisplay.php?f=22,
http://castlecops.com/forum67.html, or other appropriate forums for review
by an expert in such matters, not here.**

If the procedures look too complex - and there is no shame in admitting this
isn't your cup of tea - take the machine to a local, reputable and
independent (i.e., not BigBoxStoreUSA or Geek Squad) computer repair shop.