Have you scanned for spyware? If not, download, install,
immediately update and then run
Spybot and/or
AdAware. Read the help files to familiarize yourself with how they work. I recommend you use them both since each tends to find things the other misses. Let them clean up any spyware found.
Then, to be doubly sure, download and run
CWShredder and run it (link to the download is near the bottom of the page).