This setting is held in the registry so you'll need to run a .reg file to update the appropriate bit.
I've just had a quick browse, and the location looks like HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.TIFF. The following is an export of this into a .reg file which I've just tried and it works for me so save this in Notepad as anything.reg and see if it works for you. Note that this is on Windows 2000 Pro, so if you're clients are using different a OS, you might need to modify it.
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.TIFF]
"Application"="mspaint.exe"
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.TIFF\OpenWithList]
"a"="mspaint.exe"
"MRUList"="a"
Cheers, Antony