See UPDATE (I) below (11/24/07) on How to Configure Microsoft Office SharePoint Server 2007 to Index and Crawl Network Folders for Search

Objective: I want to execute a search restricted to a specific document library.  In this case, the document library contains training documents.

Steps:

1. Record the URL of the document library.

2. Create a scope:

Go to central administration.

Access the SSP.

Go to "Search Settings".

Scroll down to the scope section and select "View Scopes".

Create a new scope.  Give it a name and useful description.

3. Define scope crawl rules:

Add a rule that defines the document library for this scope.

Select "web address" for "Scope Rule Type".

Enter the name of your folder (see #1 above).

Accept the default value of "Include".

Wait for the scope to update (or start it manually).

4. Enable the scope in a site collection.

Go to site collection where you want to use this scope.

Add your new scope to the appropriate display group or groups.

5. Search!

At this point, you're done.  Assuming the scope is properly defined, it will be available in the scopes drop downs for simple and advanced searches and when you search using that scope, you'll just get results you expect.

Notes:

I wrote this blog entry because my searches for simple scope setup turned up dry using phrases like:

• configure scopes in MOSS
• configure search scope moss
• scopes in sharepoint 2007
• scopes in sharepoint
• search a document library moss
• scopes primer moss

You can create a search scope on a folder, not just the entire document library. 

A scope can be shared across multiple site collections (hence, a "shared service").

You can create the scope at the site collection level itself.  However, I prefer to go to central admin because I can start the crawl from there.  You cannot start the crawl from the site collection.

UPDATE (I) as of 11/24/07:

Kai Shang put together a great post entitled How to Configure Microsoft Office SharePoint Server 2007 to Index and Crawl Network Folders for Search @ http://kaishenghoo.spaces.live.com/blog/cns!8A7458DB12CA5AC9!206.entry