" I can’t find the logs at all, let alone anything about this."
Security Essentials > History > All detected Items
" I tried to update from within the settings on the XP machine where I can get into them, but (on both actually) I get an error saying that the service can’t be reached"
Right. The U3 support URL is no longer active. (This may help: http://forums.sandisk.com/t5/Cruzer-flash-drives-Solid-State/U3-COM-What-s-up-with-U3-Website/td-p/185386 ) But the link I gave you will download the last version. And yes, backup your files, AND the hidden system folder.
But the one you have with 1.6.3.9 is current and should work on Win 7 64-bit in compatibility mode for Vista SP2. Why it should give a “network” error when starting I have no idea.
hmmmm Do you have the U3 Launchpad settings set to connect to the net when starting? Automatic Updates on, Quality Improvement on, etc??
Have you tried using the U3 stick immediately after booting?
