OMG this is so strange! My brothers computer had two problems. UAC turned itself on, but the checkbox was UNCHECKED in Control Panel, and I could not turn it off. I fixed this by fixing the EnableLUA value, which had been completely deleted from the registry. Last night, I went to have a look at the Themes, and it was definitely NOT in the list. Current Theme, and Windows Classic were the only options.
So today I have more time, and I to the C:\Windows\Resources\Themes folder, and AERO is there! Odd. So I go back to the Themes dialogue, and Windows Vista has reappeared in the list! So I select it, and the problem was solved. I did nothing! I will try and help you though. Please humour me and re-check your Themes dialogue. I know it was empty, I totally believe what you said (after all it happened to me) but maybe the problem has resolved itself. If this does not work, try System Restore as suggested by Lorien, and maybe Rich (I forget who suggested it!)
If not, run an SFC scan, restart, and re-check themes. Please then follow the top yellow box, and upload the logs is Integrity Violations are found. However, no Integrity Violations were found for me, but we need to check.
Please tell me what lies in the C:\Windows\Resources\Themes folder.
Thanks!