I've been running fine since 11/11/11, but now out of the blue Skyrim won't run without running Steam as administrator. If I try on a regular account, I can start the launcher, but when I click play, it asks for an administrator login. Whether I login or cancel, it asks for the password two times, then kicks back to the launcher. Any ideas?

Move Steam outside of the Program Files directory on the system's root drive: https://support.steampowered.com/kb_article.php?ref=7418-YUBN-8129

That's the general cause and solution.

No dice. Steam lives in C:\games\steam Thanks though.

Huh. Ok, have any updates for Windows or a security program been installed around the same time the issue cropped up?