Photoshop CS3 cannot be installed on Windows Server 2003, simply because it's not supposed to be installed on Windows Server 2003.
In the Setup folder of Photoshop CS3, look for the file: Adobe CS3\Photoshop\Adobe CS3\payloads\
Note that you probably have to substitude en_US with whatever language of CS3 you're running. Open it with notepad, and look for the string that says Server2003. You'll find a line, similar to:
Change it to:
Restart setup, and you should not be able to install it.