Quickly Finding User Accounts Anywhere in an Active Directory Forest

I work in a network that contains a single forest made up of many Active Directory (AD) domains. I need to verify that a user account exists and in what domain it exists. I know I can use Active Directory Users and Computers to find an account. How can I script this task instead?
You can accomplish this task by writing a script that queries a global catalog server in your forest. Because each global catalog server in a forest contains a partial replica of all objects in all domains within the forest, it's the best place for locating AD objects such as user accounts. Listing 1, shows how you can use the ADSI OLE DB provider to search a global catalog server in a forest for a user account whose common name (cn attribute) begins with the letters eth. To make this script a bit more dynamic, I bind to the rootDSE object.

The rootDSE object contains the base Directory System Agent (DSA) information from any domain controller in the domain. DSA information includes data such as the current DNS host name and the various naming contexts in an AD implementation, such as the default schema, and configuration naming contexts. The code at callout A in Listing 1 demonstrates how to bind to the rootDSE, retrieve values from this object (in this case, the script retrieves the distinguished name of the forest, which is stored in the rootDomainNamingContext), and create a variable containing an AdsPath that uses the GC moniker. The GC moniker instructs the script to connect to any domain controller serving as a global catalog server. Under the covers, the GC moniker makes a connection request by using port 3268, which is the listening port for all AD global catalog servers. The actual bind operation performs authentication to an available global catalog server by using the credentials of the currently logged-on user. . . .