One way of achieving a no-downtime transfer is use IISSYNC which is a utility that replicates iis installs from one machine to another.
first step is replicate everything other than iis, where this is files, permissions, dsns, com objects that might have been installed from one machine to the other.
For the files, you can use the windows file replication services, though I have found this to be somewhat unreliable. There are other 3rd party file replication and syncronisation utilities available on download.com or tucows.com
If you assign a domain IUSR account to the anonymous user for IIS, then you can simply have the second directory structure exactly the same permissions as the original and when you IISSYNC it should cause no problems, otherwise you might have to sync the user accounts for each of the sites with the permissions on the files manually.
to transfer the DSNs you can simply export the registry branch called ODBC and reimport it into the other machine, beware that this will overwrite any preexisting dsns.
And then just reinstall all the COM objects to the new machine.
Once this is done, use IISSYNC to transfer the settings from one IIS to another using the syntax ;
iissync newcomputer
