I'm also having a tough time understanding what's going on, but I'll venture a guess. It sounds like one user account is running Aero just fine, while another is stuck in Basic Mode. If you can switch it back to Aero, then it is fine, and was only set that way as the default...which happens occasionally. If it keeps reverting back to Basic, you might have an issue with the graphics drivers...at least under that account.
There are some programs out there, such as Paint Shop Pro 7, GoToMeeting, etc that revert Windows 7 back to the Basic theme until they close, so it is possible this second account is running an incompatible app.