I've noticed a lot of other people mentioning variations on the runas command and how you need to know the users password, which is true, but I don't think that anyone has quiet answered the question. of "wanting the whole desktop would become the user's, etc. and not just in a cmd window". Here's the way I go about it:
Note: I'm going to refer to this first Command Prompt as CP1 to eliminate confussion later.
Under your admin account, open Command Prompt
For local account
runas /profile /user:computernamehere\username cmd
For domain account
runas /profile /user:domainname\username cmd
OR the way that I prefer it
runas /profile /user:username@domainname cmd
Note: A new command prompt will open (CP2), this is the user who are you trying to login as.
Open CP1 and type:
taskkill /f /IM explorer.exe
Open CP2 and type:
explorer.exe
Depending on the computer, it may create a profile for the user if they have never logged onto there before. You can save yourself the hassel later by keeping the command prompt windows open for later use.
When you're done with your work, just do the same thing in reverse.
In CP2 type:
taskkill /f /IM explorer.exe
Open CP1 and type:
explorer.exe
You should now be back into the orginal administrator account. You can do a quick check by tapping the Windows key and looking for the current user panel.
Hope this helped.