You need to go into the registry and delete 1.23 folders that you have laying around from a previous installation probably. If you need help figuring out what/how to remove contact me through my email, ICQ, private message, or on #jj2. I'm guessing that if you remove the 1.23 settings for list servers and user data the patch will be able to apply itself. Here are a few simple directions:

Go start->run type regedit
Remove HKEY_CURRENT_USER\Software\Epic MegaGames\Jazz Jackrabbit 2\1.23
Remove HKEY_LOCAL_MACHINE\SOFTWARE\Epic MegaGames\Jazz Jackrabbit 2\1.23

You will need to reapply the list server patch after doing this as well as the 1.23 patch.

Good luck.