"C:\Program Files\Common Files\Microsoft Shared\web server extensions\12"
In WSS 3.0, The 12 hive is directory structure where the product is installed into. This folder structure is where all the relevant web pages, features, Site definitions and web services are installed to.
Of note is the TEMPLATE directory, this directory contains all the necessary web page files that have been provisioned by default.
WARNING! You should not add to, modify or delete files in the 12 Hive. To add custom functionality to the 12 Hive (including branding) should be done through SharePoint Designer or provisioning using .wsp files.
1). Create a shortcut to this folder structure on your desktop as it is an important folder structure for you to use.
2). Add a search path by editing your environment variables to: C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\BIN