Having both on the scam computer is asking for trouble. It is not a supported configuration. Uninstall one of the apps and re-install the other. Acrobat 9 is the last version of Acrobat you can upgrade from 6 without paying full fare. Acrobat 9 has been out for some time. The clock is ticking.
I had the exact same problem.....iinstalled on a Windows XP machine
You probably had 9.3 already installed, then you pulled out your 6.0 Acrobat Professional disk and installed that.
What you need to do is "remove" your Adobe 9.3.
Re-start your machine. Then reload Adobe ...try Adobe 8.
Now it won't use Acrobat 6.0. At this point, select an existing PDF file, RIGHT click, select "Open With...". You won't probably see Acrobat 6.0 so you have to "BROWSE" ... find Adobe, then find "6.0". Select.
At this point Adobe 8 will be your default, but, you can always select 6.0 by "right clicking" the PDF first, when you want to.