You could be having malware problems. Run some (or even all) of these
tools:

Ad-Aware http://www.lavasoft.de
Spybot S&D http://security.kolla.de
AVG free antivirus http://www.grisoft.com
Etrust/Vet/CA.online Antivirus scan
http://www3.ca.com/securityadvisor/virusinfo/scan.aspx
Panda online AntiVirus scan http://www.activescan.com
Panda online AntiSpyware Scan
http://www.pandasoftware.com/virus_info/spyware/test/
Catalog of removal tools (1)
http://www.pandasoftware.com/download/utilities/
Catalog of removal tools (2)
http://www3.ca.com/securityadvisor/newsinfo/collateral.aspx?CID=40387
Troubleshooting guide to Windows http://mvps.org/winhelp2002/
Blocking Unwanted Parasites with a Hosts file
http://mvps.org/winhelp2002/hosts.htm
Links provided as a courtesy, read all instructions on the pages before
use.

When all else fails, HijackThis v1.99.1
http://aumha.net/downloads/hijackthis.zip
is the preferred tool to use. It will help you to both identify and remove
any hijackware/spyware. Post your log to:

http://aumha.net/viewforum.php?f=30,
http://castlecops.com/forum67.html,
http://forums.subratam.org/index.php?showforum=7