Hi Jane,
I don't think that downloading and running 4OD on Vista is illegal but am not 100% sure. There is a good reason as to why 4OD isn't supported by Vista (it doesn't work properly yet) but you are certainly able to try and get it to work yourself.
You probably won't "mess" up your PC by installing 4OD as per my instructions but since 4OD isn't yet officially supported by Channel 4, if you do anything that causes a crash/freeze/data loss, Channel 4 won't help you.
If you are still really serious about getting 4OD to work on your Vista PC then send me a private message with your email address and I will send along the installer (but it is all at your own risk etc...).
Thanks for the interest!
-james
Ikanos Consulting