You might be infested with malware.
What You Should Know About Spyware
http://www.microsoft.com/athome/security/spyware/devioussoftware.mspx

So How Did I Get Infected Anyway?
http://www.wilderssecurity.com/showthread.php?t=27971

Help with Hijackware
All  MS - MVP Sites.
(http://aumha.org/a/parasite.htm)
(http://aumha.org/a/quickfix.htm)
(http://mvps.org/winhelp2002/unwanted.htm)
(http://inetexplorer.mvps.org/Darnit.htm)
(http://www.mvps.org/sramesh2k/Malware_Defence.htm)

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

You likely have a corrupt cache.

Internet Explorer sets the IE cache size at a percentage of total disk size,
and with modern hard disks this is far too large for IE to handle properly
(this problem has been fixed in IE7).

So, first, go to IE tools, internet options.  Click on the temporary
internet files 'settings' button and reduce the size of your IE cache to 50
meg at the most.  Then, make sure that IE is not set to 'automatically' for
checking for new pages.  Use any option but that.

After you have hit apply once, go back and use the 'delete files' option for
the IE cache.  Enable the option to delete offline content as well.

Once you have finished deleting, shut down IE and run a defrag on your
sytem.

Then, once that is done, go back to IE and I'm betting your problems will be
resolved.