Hi eric06,
I am running the configuration you are talking about and everythings working fine.
You are talking about two different problems:1. Roundcube is not running
2. Subdomain points to standard plesk webmailer (maybe Horde), not to Roundcube
solutions:1. In standard windows configuration plesk is installed with the mailserver mailenable standard. This mailserver doesn't support imap and Roundcube only works with imap. I have disabled mailenable and installed the free version of hMailserver for windows. It gets fully integrated in plesk, imports all your mailaccounts and supports imap
For installationguide ask google
2. You must save your RoundCube installation somewhere in the inetpubfolder. For example next to the Horde-Folder. Then go into InternetServiceManager and change the virtualWebsite-Directory to your Roundcube-Folder.
I hope this helps and sorry for my bad english
greets,
DocNight