Quote - No software is safe from assault.  The best, strongest, most invested-in protection cannot be totally secure against attack ever.

100% secure is impossible, but you can achieve very high levels of security.
Rule #1
No remote execution of code is allowed.

With only folowing this rule:

• No matter which site do you visit there's no way to a virus or spy be installed in your computer.
• No way to a hacker can enter your computer through the internet.
• No virus can be installed by a received email.

But if this rule is followed:

• No automatic upgrades to your computer.
• No way to Micro$oft modify your computer online.
• No activeX.
• No way for companies install toolbars, search engines and propaganda popups into your computer.

The only way to have your computer infected is to you by yourself run an infected executable file, downloading a virus is innofensive, you must execute it !!!
As remote code execution is not allowed, the only way to execute a virus is by your own hands, of course you can be cheated by an email.
"This is funny, click here: funny.exe"  (no hidden extensions allowed, allways appear .exe and exe is the only executable allowed (no .scr, .pif))
After clicking on "funny.exe" will appear a message: "do you want to install"/run this program?"
If you answer yes, the problem is yours, security has it limit and the limit is stupidity.

Stupidity also evolves!