Acrobat asks for serial number, but why ?

Hello!

First off all it's the first time i'm here on the forum and of course i have an issue with my CS4 standard.

Of all the applications only Acrobat wants me to re-enter the serial number.
If entered, it says it's already in use.
Does anybody know how come?
I cannot use Acrobat now.

I deinstalled Acrobat and want to reinstall it again, but now the disc doesn't want to install it!
It's taking long time with the message "preparing to install"

I am now out of options, who can help me?