Create a plain text file called ContentBrowser.ini and save it to the root folder of your AutoCAD Architecture installation, which is the folder where the acad.exe and AecCB.exe files are located. (The default location for Windows XP is C:\Program Files\AutoCAD Architecture 2008; your location may vary.)
Place the following text in the file:
You will want to substitute your path and CBL file name for C:\ContentBrowserLibraryFolder\ContentBrowserLibrary.cbl in the example above. Do not forget to include a carriage return (hit the "Enter" key, for those of you too young to have used a typewriter) after the last line.
Ordinarily, this should not be necessary, but I can think of at least two situations where this would be handy. One I experienced myself, when testing out an early version of a deployment for 2008, in which the Content Browser Library file location was set to a network folder that was read-only to me. This has been corrected, but at the time, I had to manually point Content Browser to a library file on my computer each time I opened it, in order to be able to open/create catalogs other than those set in the network file. I wish I had known that I could have redirected the default location this easily.
I could also see a situation where, in order to cut down on the number of catalogs that have to be waded through to find the one of interest, separate CBL files might be used. The ability to assign types to catalogs and sort/filter them by type would be my first choice if I were using a later version, since that is much easier to manage than editing (or swapping out) the ContentBrowser.ini file.