Here's the database collocation statement from the OCS documentation:
Collocation of the archiving database is supported on both Standard Edition servers and Enterprise Edition Front End Servers; however, we do not recommend collocation on Standard Edition servers because of the potential impact on performance. Collocation of the Archiving and CDR Database with the Back-End Database for an Enterprise Pool is not supported.
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But because mixing SQL Server Standard/Enterprise with SQL Server Express on teh same host is a bad idea it's much better to install an additional SQL Express instance on the Standard Edition Server to store the Archving/CDR database.
For R2 the same approach can be used for the Archiving Server database. You can manually reinstall SQL Express 2005 for it's own setup files (not the OCS setup wizard) and install a new named instance. I beleive up to 16 instances are supported for SQL Express on the same host.
Jeff Schertz, PointBridge | MVP | MCITP: Enterprise Messaging | MCTS: OCS