Rename the HOSTS file.
C:\Windows\System32\Drivers\Etc\HOSTS
next:
If you are using Internet Explorer, I would check what add-ons are running and disable any that look suspicious. In Internet Explorer 6, go to Tools and select 'Manage Add-Ons'. Then scroll through the list of enabled Browser Helper Objects (BHOs) and disable the ones you don't want. In IE7, go to Tools and Then Manage Add-Ons and then Enable or Disable Add-ons to do the same.
If the issue still continues, your situation may be due to Spyware, Adware or Malware.
Start by emptying your Internet Cache*.
Next, here are excellent free tools for removing spyware, malware and adware:
Ad-Aware:
http://tinyurl.com/5kgsl
Spybot Search & Destroy:
http://tinyurl.com/78ubw
CWShredder:
http://tinyurl.com/5lstv
X-Cleaner Micro:
http://tinyurl.com/357tuu
I would also use this tool: RootKitRevealer (http://tinyurl.com/y3hgq9)
If the issue remains, I recommend you download and run the latest version of HiJackThis (http://tinyurl.com/b9h3s) and post the log file it creates in one of the many forums dedicated for that purpose. I recommend that you post your HiJackThis log where there are many people who specialize in working with them. Try here: http://tinyurl.com/2nxzqt
You'll have to create a login but I think you'll find it to be the easiest and best way to resolve your issue.
Additionally, here is a good source for advice: http://aumha.org/a/quickfix.htm
When you finally get everything cleaned up and running smooth, here are great free tools to help you avoid getting that garbage again:
SpywareBlaster:
http://tinyurl.com/g1d9
SpywareGuard:
http://tinyurl.com/3yj37
and to quickly clean out the places where their installers usually hide before you shut down or reboot - ATF-Cleaner:
http://tinyurl.com/2clx6g
Good Luck!
** To empty the Internet Cache - For IE7, open to a browser window, click on Tools, select Internet Options, in the Browsing History section click on Delete and then in the next menu click on Temporary Internet Files 'Delete Files.'
If you're nervous about removing things found by spyware scanners, you can always create a System Restore Point as a safety net.