Your situation may be due to [b]Spyware, Adware or Malware.[/b]
Try emptying your Internet Cache*
Here are excellent 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
If the issue remains, I recommend you download and run 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.
When you finally get everything cleaned up and running smooth, here are great 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/kqmvp
On the other hand, if you believe your issue is related to a [b]Virus or Trojan...[/b]
using AVG Free (http://tinyurl.com/2mlwm7) or your own anti-virus program, with the latest virus definitions, perform a full system scan in Safe Mode. You can also use McAfee Stinger (http://tinyurl.com/1x4v).
I would also use this tool: RootKitRevealer (http://tinyurl.com/y3hgq9)
or AVG's free RootKit detector and Remover: (http://tinyurl.com/35jq2t)
If you determine that there is indeed a virus involved,
I recommend you go here:
http://tinyurl.com/hnsvd .
TrendMicro has always been my favorite source for virus information and removal.
My second choice would be BitDefender. Here's a link to their extensive library of free removal tools:
http://tinyurl.com/y6opnp
Please let me know what ultimately resolves the issue.
Good Luck
* To empty the Internet Cache - if you are using 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 are using IE6, click on Tools, select Internet Options and then under Temporary Internet Files click on the Delete Files button.
If you're nervous about removing things found by spyware scanners, you can always create a System Restore Point as a safety net.