Are you just trying to run the trial as a trial user, or is this a fully purchased version, academic, or what?
You might try creating a virtual machine and installing it on there and then just copying the folder it installed to to your main hard drive partition and try running photoshop from there and see if it'll fix up the registry or whatever when you run it. IIRC it keeps a lot / most of its files in its main installation directory.
Did you ask Adobe tech support? Mostly I've found them annoying and almost useless though mainly what I resent personally is NEEDING to keep calling them because of a stupid bug in their installation/activation stuff that affects me for which they don't seem keen on offering a work-around for so I waste 30 minutes on the phone every single time I reinstall for no good reason.
I have the impression that they might be more helpful for a "less stupid" origin technical problem with their software, though I suspect you'll burn through about 2 hours on the phone and be told to do stupid things you've already done like "uninstall then reinstall", "disable your anti virus software", "reboot the pc", et.al. before anything possibly genuinely new / helpful may be suggested.
I suppose if they totally can't help you I could try to help do a differential analysis of my working installation against your failed one and see what's going wrong.
I think the installer may create some logfiles about its installation actions and successes and failures therein -- I'd HIGHLY commend finding and looking at these log files for any signs of failure or error messages or whatever. Also check your windows event logs for any warnings / error messages.
What vista are you all running? Did you try installing it to a different directory than the default one?
OH YEAH -- there is a documented problem in CS3 I think it is the CS3 patch specifically that says that it will give an error and not be able to "find" the CS3 install if you have installed it with a different directory name than its default... I believe you CAN move it outside of C:\Program Files\Adobe or whatever to somewhere like E:\PhotoshopInstall or whatever, but whatever comes NEXT in the "default" path uh.."Adobe\Photoshop\CS3" or WHATEVER it is is a part you cannot change to something else, so keep that part down to the end of the path the same as the default. Read the release notes for the patch it'll talk about that.