SP Execute Permission Denied To Windows Group

Hi

I'm fairly new to the world of SQL security (primarily a VB developer) and have a couple of questions/issues to resolve:
The error I'm getting in my app is "the execute permission was denied on the object 'MySP'...schema 'dbo'". In essence I cannot execute any stored procedures when logged on as a non-admin user (all this was of course fine in my development environment, now I'm hitting the security issues!).
My security is set up as follows:


Using Windows Authentication

I have an Active Directory Group "Group1", to which my non-admin user "User1" belongs.
In SQL Mgt Studio I created a login for Group1 under Security/Logins
I then created a user under MyDatabase/Security/Users, called "MyDbUser1". This uses the login above.
All SPs are owned by dboSo what am I missing?

I'm running SQL Express 2005 SP2 on W2003

many thanks

Richard