1. Delete your temporary internet files and all files in your temp directory first.
2. Boot your computer into safemode.
3. If you don't all ready have virus software get some, run the virus software, delete the viruses that it finds.
4. See if you can get onto the internet after deleting what viruses it picks up with the base dat engine that came with the antivirus software.
5. Update your antivirus dat engine.
6. Rerun your antivirus software and delete the viruses if any.
7. Get the critical windows updates for your version of windows from
http://windowsupdate.microsoft.com/
EDIT:
You might also have spyware on your PC, check out what your homepage is set to. If it is set to something other than what it should normally be than you might have spyware. Do you recieve a ton of popups? Does your computer seem much slower booting up than it did a while ago?