Safari corrupted?

recently whilst surfing I was advised by an official "mac" looking pop up that i needed to upgrade in order to surf to Ebay. Stupidly I did and since then on launching Safari whilst it initially goes to my homepage it will almost immediately trip to other sites making offers etc and dire warnings of security breaches on my computer.

i have tried to delete Safari in order to reinstall however it will not let me as it is part of OSX. I have tried restoring from my original OS X Leopard install discs but it doesn't let me because "Power point " is not supported. So I am stuck - and not being a very computer literate person do not know what to do next.

I wonder if anyone can help with some simple advice?