Has anyone seen this error on an XP desktop machine before?
'net' is not recognized as an internal or external command, operable program or batch file.
It's being used in a 'net start iisadmin' script
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.
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.