Want to keep hackers from getting into your computer and its data? Follow these simple steps:
1) Open your computer's case
2) Wire up some dynamite, place inside of case and as close as you can to the hard drive(s)
3) Hook detonation wire up to detonator.
4) Place yourself a safe distance from the computer.
5) Activate the detonator, and watch.
Well, I didn't say this was PRACTICAL.
EDIT: I just want to comment that you cannot have perfect security, though. If you want to secure your system, you need to keep your virus protection and firewall up to date. Either that, or just ditch Windows, because it is one of the least secure operating systems. You're probably better off running Linux, Mac OS, Solaris, or FreeBSD. Even with those operating systems, however, you cannot have perfect security, but at least your out-of-the-box security will be better than a Windows system with an up-to-date firewall and antivirus.