Vista Tutorial - VS2005 security - domain users can't start VM's

Hi,
We've recently deployed VS2005 onto Windows 2008 64Bit and (after much
trial & error as well as web searching) we've managed to get to a point where
the VMRC client will connect to the new VS installation and users can access
the running machines.

The big problem we have however is that anyone who isn't an administrator
of the host machine isn't able to start a VM! The error message which is
displayed in the Virtual Server Event Log is:

"<VM Name>" could not start because it is being used by another program, or
marked as read-only.

I've triple-checked the file and folder attributes and the files aren't
read-only or in use (that I can tell). This also doesn't seem likely as an
admin can start a machine which a domain user can't.
The permissions on the folders containing the VM's has been set to allow
Read, Write and Modify access to our Domain Users using a security group
which everyone's in.

I don't want to add everyone as a local administrator of the server but
can't currently see any other solution. For reference, the entire error
message is pasted below.

Thanks in advance for anyone's input,
Andy


Log Name: Virtual Server
Source: Virtual Server
Date: 16/02/2009 09:59:00
Event ID: 1067
Task Category: Virtual Machine
Level: Error
Keywords: Classic
User: DOMAIN\username
Computer: SERVER.Domain.co.uk
Description:
"VM Name" could not be started because it is being used by another program,
or marked as read-only.
Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
<System>
<Provider Name="Virtual Server" />
<EventID Qualifiers="49410">1067</EventID>
<Level>2</Level>
<Task>2</Task>
<Keywords>0x80000000000000</Keywords>
<TimeCreated SystemTime="2009-02-16T09:59:00.000Z" />
<EventRecordID>476</EventRecordID>
<Channel>Virtual Server</Channel>
<Computer>SERVER.Domain.co.uk</Computer>
<Security UserID="DomainUser's SID" />
</System>
<EventData>
<Data>VM Name</Data>
</EventData>
</Event>