One - WinXp64 was an oddity, that is not supported by much
Two - "When run under the administrative account, the program seems to function normally"
My understanding (remember that this is a USER TO USER forum, so I don't work for Adobe) is that all flavors of Premiere require 100% of a computer's resources... and that includes being run as Administrator
If the program works for you when run as Administrator... that is what you need to do
Three - "My Documents is redirected to a network share, in case that would be an issue"
Again my understanding... all flavors of Premiere are NOT designed to use a network... they were/are designed to be installed on a single computer and to run on that computer
Assign 'Full Control' permissions to 'everyone' on all folders used by PRE (program, project, scratch)
Insanity is hereditary, you get it from your children
I would definitely look to the Administrative Account, and make sure that the program is installed locally, and not on a networked drive. This CHECKLIST is for the general installation of Adobe products.
As has been pointed out, XP-x64 was never a certified OS for any Adobe program. It was basically a beta-only OS, and in many ways an evolutionary dead-end, though I am sure much of its code survives in Vista and Win7. Many Adobe users had issues, but many got it to work fine. Much seems to depend on the various drivers.