Problems re-installing after on-line updates cause version change from install CD

Question

I'm running AcroPro 6 updated to 7.1 through on-line updates, on laptop and was on desktop. The laptop runs just fine with no issues, and Acrobat 6 does everything that I need done, so I don't have any desire to upgrade to the latest and greatest. Problems with Acrobat on the desktop caused me to uninstall it, and I couldn't reinstall it.
Now, using the laptop in conjunction with the desktop isn't the most convenient because I'm creating documents on the desktop, and then have to port them to the laptop, do whatever is needed there in Acrobat, and then port back. Usually all that I need to do is to assemble a pdf from separate pages, insert or remove pages from a pdf, or to apply security. Nothing particulary fancy.
Acrobat on the desktop worked fine for many years, but at some point reported an issue finding the printer, and refused to print anything. After trying everything I could think of as a possible solution I, perhaps foolishly, uninstalled Acrobat with the intention of reinstalling it and having the problem resolved. On attempting to reinstall, starting from the original Acrobat 6 CD, the installer reported that a later version of the software was installed, and exited. It would seem that simply uninstalling Acrobat leaves sufficient references in registry or other places for the last update applied to be determined during installation.
There seem to be two possible options so that I can avoid the, to me, unneccesary expense of upgrading to the latest version of Acrobat.
Locate and remove the source of the data that the Acrbat 6 installer is reading and which causes the installer to exit, or
Slipstream the updates (to 7.1) into the Acrobat 6 installation files, so that the version I would be attempting to install is the same as the verion that I uninstalled.
I tried scanning registry to find all references to Acrobat and or Acrobat 6, but I'm not certain this is the best way. Missing one might be critical, and removing them might just cause worse problems.
I tried slipstreaming the Acrobat 6 installer, but it fails using the msiexec /a switch, as it appears that Acrobat 6 wasn't designed to work with an administrative install point, according to the pop-up dialogue box.
My problem is that while I just want to reinstall Acrobat 6, update to 7.1, and go on happily, I can't see how to do this. Can anyone advise if there is a solution other than upgrading?
Thanks in advance for you thoughts, all will be appreciated!

Answer 1

Thanks John, Bill and Graffiti for your interest!
The original install, many years ago, was of an education version from a local college. As updates became available thorough the update manager, they were installed. As stated above, it updated though 6.0 to 6.6, and then on to 7, eventually to 7.1! I can only repeat what happened, but not explain it. I thought nothing of it at the time, as it was all being managed by the update manager. I agree that this last seems to be more an upgrade than an update, but... what can I say? What happened is what happened, and it happened as I described it.
But not's get hung up on the history! Now that I seem to have solved the original issue of not getting past the message that 'a more functional version has been detected', how does one solve the 'invalid serial number' issue that occurs when attempting to reinstall 6.0? Delete the Acrobat registry keys from HKLM and HKCU, and possibly anywhere else? The former didn't work yet, so, where else? Maybe there is an 'update' folder hanging around somewhere with a registry reference that needs to be deactivated?  Something like {AC76BA86-0000-0000-7AC5-6028747ADE00}?
Any further thoughts? And thanks for the input, it really is appreciated!