[{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.

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 !!!
 
There have been cases of published software, by major vendors, having viruses. There are also other attacks (such as buffer-overflow attacks) which do not require the user to initiate the remote execution. Also, the level of lock-down required to avoid all possible methods of insertion of viruses is sufficiently draconian that it is infeasable for most home users.