It is essential to check for spyware/hijackware/foistware.

Go to IE tools, internet options, general tab. Click on the cache settings
button and then 'view objects'. Delete anything you don't recognise.  If you
are unsure, or no objects appear, for diagnosis purposes I REALLY like
BHODemon, available at http://www.definitivesolutions.com/bhodemon.htm.  It
does not need installing - simply unzip and run the EXE programme.  It is
very easy to use.

I find this programme is a better option than IE6's ability to turn off
"Enable third-party browser extensions (requires restart)".  This disables
*all* plug-ins and makes troubleshooting very difficult.

Many people like AdAware, available at www.lavasoft.de .  Make sure you keep
the signature files up to date and remember, AdAware may only remove the
current installation of spyware; it may not do anything about software that
reinstalls itself, so unless you want to get stuck in an endless loop of
hijack/cleanout/hijack/cleanout make sure you get rid of whatever is
installing the junk.  See my Troubleshooting advice for information about
how to track down and get rid of spyware completely.
http://www.mvps.org/inetexplorer/Darnit.htm#tshoot

An excellent replacement for AdAware is Spybot. Again, it is a free
programme which can be downloaded from:
http://spybot.eon.net.au/