Normally I use Outlook 2007 for an e-mail client, but recently I have tried the beta of office 2010 untill the license expired.
I reinstalled office 2007, but now I get this annoying window at outlook startup telling me that the user profile was made in a newer version of Outlook, in which incompatible information might be stored with the present version. If You experience problems with this profile You can make a new profile.
Now I have absolutely no problem with the profile. If I click o.k. I can use Outlook as usual.
How do I permanently get rid of the annoying warning?

You reinstalled the Outlook 2007 without removing of 2010 beta?
The profile of 2010 beta stay configured?

If yes, then install the Outlook 2010 and completely remove it, after that reboot the computer and install Outlook 2007. Don't forget to make a backup copy of your PST file just in case.

I removed office 2010 before installing 2007 using the normal procedures. To make sure I will remove 2007 again reinstall 2010 and then remove it again before reinstalling 2007.

I just figured out what to do. It is quite simple.
First save all your mail and contacts on a different partition.
Go to start, control panel, E-mail, Profile, remove profile.
After that you have to configure outlook again.
Then import your mail contacts and other data.