The only truly secure computer is one locked in a room with no network access and no mouse or keyboard.
Just about all security measures boil down to security by obscurity, meaning people can't hack what they can't see.
Assuming you plan to some version of Windows the best you're going to get (on an end user budget) is:
- a decent firewall (Cisco PIX 501/SMC Barricade)
- antivirus software
- trojan removal program
- Windows 2K/XP with NTFS perms and apply one of the security templates.
- create a non-admin user account with a good password and use that for your daily usage. Virii tend to run wild on Windows because people use the computer using the Administrator acct which has the power to do anything, that means the virus you let execute can do anything.
If you're that serious about you should at least look into some user friendly version of Linux like Red Hat, Mandrake or Suse. I personally use FreeBSD and I don't give security a moment's thought most days. I have my own OpenBSD firewall w/Intrusion Detection , spam protection & antivirus.
95% of hackers out there are the kiddie script variety. They're easily stopped with most any up to date firewall & AV. The next 4% are the ones you worry about. Make yourself a difficult enough target that they'll just move on to someone else. The last 1% are going to get in no matter what you do; you just have to hope they have bigger fish to fry than you.