not much to go on,could be anything.Check for virus, trojan, spyware,etc. Then you could try sfc /scannow from the run command. (have win xp/cd handy)
Also if "no virus or trojans" try a system restore.(should be disabled if you have virus etc.(if trying to fix if any)
If above fails suggest you create another account before you delete your original account.
Copy your favourites folder for IE info (If required) for to place in new account,then delete your original account.
Your new created account and all the setting will be as standard and you may have to input some software product ID Serial numbers when you first startup for some programs(depending on what you have )