get 'sweeper' by emery, freeware
http://www.bmesite.com/
this can clean your index.dat plus many other stuff, cookies etc It may not cure your problem but it gives your pc a good clean. this freeware is safe i use it every day.
Second, if you got XP, press Ctrl-Alt-Delete and click on 'processes'; print a list or type into notepad go to
http://www.answersthatwork.com/Tasklist_pages/task
list.htm
where you can find rogue processes running on your pc. That should solve your problem. ELSE; run regedit and 'export all' to My Documents just in case u crash your system you can import your registry back to how it was. While running Regedit, choose 'Find' and search for a likely Key word e.g. 'sex' or whatever you can think of; hopefully you will find the fiend and delete the offensive 'keys' from your registry.