ASP Security causes QueryInterface InvalidCastException?

[Macca]

All,
I have a vb6.0 com+ proxy component which works fine from
a vb.Net console app.

If the same code is placed into a ASP.Net web service I get
'QueryInterface for namespace._class failed'

I can see on my 'application server' that the COM+ object
gets instantiated ok. But when I attempt to call one of
the object's methods the exception is thrown.

Microsoft have suggested that this could be an ASP.Net
security issue. (hence this ng) I've checked IIS, NTFS and
COM+ authentication so that a user has sufficient access
rights but other than this I'm at a loss as to what to do.

Any suggestions greatly appreciated.
Regards.