"I think it has something to do with Administrators and UAC but can't find what............"
I think you may be looking in the wrong place Jim.
The settings that control file sharing in Windows 7 can be found via Control Panel -> Network & Sharing Center -> Change Advanced Sharing Options. See if you can spot any differences between your two Windows 7 computers and make any appropriate changes. The "Password Protected Sharing" option may be the culprit here although I would have thought you'd get different symptoms.