Do you have a working version on any machine? If so, you could try copying the whole folder as I've done.
Edit
Quote: "Good luck."
Thanks. Just tried installing directly from the DVD and things are working fine - but I did cheat a little.
This is what I did:
1. Insert the DVD (but took me a while to locate the case for the game dvd that was already in the machine

) and ignore all the prompts to use the builtin setup procedures.
2. Browse the DVD and find the TreeMagicG3 folder.
3. Copy that folder to the desired location on my PC (I used My Documents as suggested earlier).
4. Open the My Documents version of the folder, find the file
TreeMagikG3_107.exe and install to your desired location in the usual way.
5. Run TreeMagik.exe.
I guess you could probably run that file directly from the DVD but since the above works and takes only a few seconds I can't be bothered to test. Similarly I haven't tried installing to the default Program Files location as that will probably end in tears.
I haven't tried installing to a memory stick though.
I now have two identical copies of TMG3 on this W7 machine and they both work perfectly.
Edit2 And just to rub it in

: