Things randomly changing in your browser isn't likely a hacker, it's more likely some kind of spyware or virus that's automatically doing it. I don't know if anyone would waste their time doing something that ridiculous to you by hand. Try different spyware-removal programs, not just one, because some of them find different versions of spyware than others.

Trojans = bad though. The only sure way to remove viruses is to format your hard drive (i.e. completely erase everything) and reinstall Windows from scratch. If you have a trojan, it's definitely a good idea to do that, because who knows what's going on if someone is backdoored into your PC. Virus detectors / cleaners are handy, unless the virus happens to mess with the virus detection program itself, so even that isn't a sure way to get rid of one. Just my opinion though, I'm admittedly a bit more paranoid than most people.