It is impossible to guarantee outbound protection on any Windows XP computer. One a file has administrative access it can get around any "outbound" protection at will.
Agreed, but that's no reason for not using outbound protection.
Analogy is using an antivirus, it's no guarantee againt viruses at all.
This is another layer of defence and the most impotant tool is still your brain.

I regularly take a 'known to be good' image of C to another partition.
In addition I hide that partition from me and windows. Yes, I have imaged back
a lot of times to completely undo things I have done.
But guarantee... no, just another level of protection.