"su - user" has existed for at least 30 years.

Oh, right, Windows. Using Windows as a counter point to any argument on best practices is silly. But even then, as your domain admin, I can reset your password, login as you and then reset a new temporary password for you. I have done this and would never ask a user for their password. Even on windows.

1. I wrote service as in service provided on the web, not software. I'm sure online office (360, live, or whatever the name is this week) support has the ability to get to your documents though.

2. There's runas.

3. There's lots of examples of software that allows you to use the system remotely. Some of them have signatures in the virus databases, some don't.

runas.exe: http://technet.microsoft.com/en-us/library/bb490994.aspx