CS3 windows version how can I run it on an iMAC

You can also run Parallels which would give you access to Windows applications from within OS X.

(This question is not about the Photoshop SDK, which is a tool for
programmers. I will answer anyway, but please post future questions in
a more relevant forum, thanks! ...)

You certainly can't install Windows software directly on a Mac. If you
have Lightroom which runs on either platform its because you have
installers for both Mac and Windows software, which is not what you
get for all programs on the CS3 disks. With rare exceptions you would
need to buy ALL your software again, unless the vendor offers both
versions on one disk, or a way of upgrading between PC and Mac.

However, if the Mac is running Leopard, you can buy and install
Windows XP (full retail version, using a disk which came with a PC is
NOT allowed) and then boot up Windows in place of Mac OS X. Then you
can run CS3 for Windows, and any other of the Windows software you
don't want to buy.

I wouldn't personally recommend buying Vista instead.

Aandi Inston