Originally Posted by Wheatfield 
It's a bit worse than that, actually. Failure to deactivate the software prior to uninstalling it causes you to lose the license for that software. I had a bugger of a time with CS-3. I had put a legitimate copy of it on my workstation at the studio. One of the photographers came in with a hacked copy of CS-4 which they installed, and then they uninstalled my CS-3 without deactivating it and bam! I'm down to one license, and Adobe can't seem to do anything about it. I had to take my home machine into the shop, and for some reason CS-3 couldn't deactivate from my machine and bam! all of a sudden I have no active licenses for CS-3, so when I want to install it, I have to call Adobe and go through a beg and plead song and dance to get it working.
My CS-4 upgrade went smoothly, you put in the serial number very early on in the process and then it installs. Activation takes place immediately upon the completion of the install.
This why I image my hardrive regularly or prior and after any major installations. You could have "restore" which would or may have solved the problem. Ideally, imaging your drive with incremental regular updates is the the best solution. I pay $60 for the software which is used to image 2 PCs and 3 laptops.