Doing a system restore is over kill, you just need to reinstall whatever it is you uninstalled.
What do you actually mean by "unistalled yahoo" though? Do you mean the Yahoo Messenger chat program, the Yahoo toolbar for your web browser, the Yahoo web browser that comes with some Internet Service Providors or the Yahoo Commicator that comes with BT broadband?
If you uninstalled Yahoo Messenger go to http://messenger.yahoo.com/ then click on "Get it now" and follow the insructions.
I suggest putting more details when you want help from people.
Jim Bob
September 2006