Have you tried reinstalling Windows? You never said which OS you are using or any real info about your computer so it's hard to help you.
Help!!! I have a laptop which has just received a brand new hard drive, as old one died. Stupid me allowed a trojan to infect my new system so I tried to do a system recovery from recovery disc. I have sucessfully opened and reloaded new system and tools, but the old one still exists when I startup. I feel that it may be having an effect on my system running. I have tried to reinstall Firfox on new system and its not working. Please help I'm not computer saavy.