The imp.reg does half of what I need. It does make IMP the standard email client, but it's not able to make IMP the default mailto handler, meaning that when I click a mailto-link OE still pops up. After googling some more I found this:
There is a different key in the Registry, referred to
by MS as "default mailto protocol handler". Its at
[HKEY_CLASSES_ROOT\mailto\shell\open\command].
You have to edit it manually using Regedit.exe.
I pasted the "iexplore.exe -nohome
www.mysite.com/horde/imp/?actionID=login_compose&to=%1" command from imp.reg into that key. Horde does start up then after clicking a mailto, but doesn't start composing a new message...
At the moment I'm stuck a bit.
Oz