You are talking about usb devices.
A good start would be installing McShield if you don't have it already.
Ok, then I don't understand, why Avast removes a 10kB-option in a 685MB program (size of Avast program folder), just to open a security risk...
What has that got to do with registration ?
Ofcourse size changes as avast (as well as many other programs) are using temp files.
It has nothing to do with a security risk.
You have a system that is conencted to the Internet.
If you believe that system is properly protected, use ICS to connect the XP system to the Internet through that system.
The first system will "protect the XP system"
If you want more security, get and use a good hardware firewall and put a av on the router.