Error: "'net' is not recognized as an internal or external command, operable program or batch file"
Solution 1:
Sounds like your 'path' environmental variable is getting stomped on somewhere. The fix is to do a direct call to it like has been suggested above.
Solution 2:
Try looking for a file called net.exe in C:\WINDOWS\system32\
If it is there then try using an explicit path to the file in your script.
E.g.:
C:\WINDOWS\system32\net.exe start iisadmin