Execute Powershell commands remotely

Try using the format: powershell.exe -Command "& {<command>}"

This worked for me, using your example:

powershell.exe -Command "& {(New-Object System.Net.WebClient).DownloadFile('http://google.com/robots.txt','c:\robots.txt')}"

Also, I used single-quotes in DownloadFile

Reference: http://technet.microsoft.com/en-us/library/hh847736.aspx