Windows 7 has impressed me, with one exception: it periodically logs in using a “temporary user profile”. This seems to happen only after a previous user has logged off.
Various people around the Interwebs have had the same problem. The only firm answer I’ve seen so far is that it appears to relate to Google’s automatic updates services for Chrome (and possibly other software).
So if it’s happening to you, get into the list of Services, and disable anything to do with Google updates. Seems to work for me — though at one point I thought I had it licked, with the Google Update Service disabled, but it started happening again. I took another look and from nowhere, the Google Software Updater had arrived on the scene, and had to be disabled separately.
(I wanted to post a picture of the error message, but that, like everything else to do with the temporary profile, has now disappeared into the ether.)