I am pretty sure you have a browser hijack or some kind of malware.
Try this free browser hijack removal tool.
http://download.cnet.com/Browser-Hij...-10564949.html
It may be a little easier to use than HijackThis. Note; you have to select for removal items found after scanning with HijackThis. Use the information tool to identify the items that you can remove safely if you use it.
Also; search for Malwarebytes and download/run it. It is a fabulous freeware program.
I like cwshredder for cleaning temporary files and cleaning system registry.
If you are not using Mozilla Firefox for a web-browser; I would look in to downloading and using it instead of Internet Explorer. Just a preference thing for me.
Good luck...
*I hope you have an anti-virus program installed and running on your pc. If not; check out the free version of AVG. Note; be sure to boot in to safe mode (hold down F
during startup and run your complete virus scan in safemode. This helps to thwart some creative viruses that can hide and redirect your scans. Plus; it will speed up your virus scans significantly.