it didn't work for me. am using win xp SP3
Does not work... don't waste your time!
I have it working with a modified version of these instructions. ZD was already set as the default mail application under ZD preferences but it wasn't working. In the Win7 control panel default it also never appeared as a program option and was never associated to mail.
What I did was clear out all the values in the first section (registry mailto), closed the registry editor, then went into ZD and set it as the default mail application (the checkbox had unchecked itself). I then checked the registry and the registry mailto section now had Zimbra values populated. Also the Win7 CP defaults now have ZimbraDesktop listed.
Without restarting, Firefox is opening ZD correctly. This is Win7 64bit.
BTW in craftedpacket's solution he just missed out "Software" in the registry but I didn't need to touch this. HKEY_LOCAL_MACHINE>Software>Clients>Mail
Did you follow the instructions exactly as indicated? What version of XP are you running, is it professional or home? I did mine in professional. Also look at the update done by Markpr in this thread.
Sorry for updating a long dead thread, however in my experience with mailto: urls failing to open in Zimbra Desktop, I was able to fix the problem by updating
Originally, even after setting Zimbra Desktop as the default through the preferences, this stayed as Thunderbird.Url.mailto."Progid"="ZimbraDesktop.Url.mailto"
Manually updating it resolved my issues.