Photoshop CS3 and CS4 Conflict

Since a lot of my friends make graphics a couple months ago I decided to try my hand at it. My first instinct was, of course, to download the leader in photo editing software, Adobe Photoshop CS3. I did this because although CS4 was out it was difficult to obtain with all the questions Adobe wanted answered. So after downloading the CS3 trial I found I was able to download CS4. WITHOUT uninstalling CS3 (32 bit) I downloaded CS4 (64 bit) to my computer and while I was downloading one and running the other something went horribly wrong. My registry got all messed up and I had to do a system recovery, which sucked. Now I really want CS3 back but I ike the 64 bit of CS4. Can anyone here tell me if there was a software conflict between the two that caused my near system crash? Any help is apreciated.

If this matters, which I don't think it does, I have an HP laptop running Windows Vista Home Premium x64 SP1.