Hawkeye just had a minor typo and IsOs's command WILL work IF the path to deltree is set beforehand, or if, instead, you type out the complete path to the command. Here's how I do it:
c:\windows\command\deltree /y c:\windows\temp\*.*
This assumes you've installed Windows to the default c:\windows folder.
You can, as IsOs did, leave out the "\*.*" because Windows will automatically recreate the c:\windows\temp folder at startup if it's missing. That's just a matter of choice. The difference between IsOs's version and mine is that mine empties out all files and subfolders of the c:\windows\temp folder, while his deletes the Temp folder itself and all it's files/subfolders.
I also use the line:
c:\windows\command\deltree /y c:\temp\*.*
because I have a c:\temp folder in addition to the c:\windows\temp folder.