Copying Certificates from the Trusted Root certs store to the Personal Store on XPsp3

[Uncle_Nick]

I have successfully distributed a couple of private certificates by GPO to a set of machines in an OU.
[ The certs were created with MS Office MakeCert ]

GPO puts the certs into the container Computer \ Trusted Root Certificate Store.

In order for users to trust the signed code in an Excel Macro, they are still being asked to INSTALL the certificate when opening the spreadsheet.
The only apparent effect / benefit gained from the GPO distribution is that the cert properties show the cert is trusted, rather than unknown.

Is there a sensible method for distributing certificates to users so they do NOT have to manually install when they first encounter the spreadsheet ?

I have identified that the MS tools CertMgr and CertUtil both have the capacity to extract certs from stores and add certs to stores. This would be great, but:
Does anyone have a comprehensive list of certificate store names ?
Using an XPsp3 PC, an arbitrary Certificates MMC covering Local Computer and Current User exposes some 11 cert stores, but does not reveal their names..... Extensive research has harvested 4 storenames so far:
- root, ca, trust and my

CertMgr has a flag -s to indicate a referenced store is a system store [presumably system==Local Computer, as opposed to Current User] whilst CertUtil has a -user option which probably performs the complementary differentiator function.

Please could some clever person offer assistance ?

cheers
Nick