I DID IT!
http://webmail.flexza.netlet's do a quick tutorial
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1) Install Roundcube in a folder on your server and be sure that it works. How to do this is in the manual of roundcube, not gonna explain it.
2) Backup /usr/share/psa-horde to /usr/share/psa-horde-BK
3) Make clean dir for /usr/share/psa-horde
4) Move all files from the folder on your server to /usr/share/psa-horde
5) Go to webmail.domain.com and it will work!
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I did this with plesk 8.2.0 on a server runnin Suse Linux 9.3.
The only issue I've got is that the button called webmail in plesk won't work anymore...I just ticked the box off at the mail preferences
Don't know if it's as simple as this on windows.
If you won't succeed just contact me @ flexza[at]flexza[dot]net