Where are iPad backup files stored on my Windows computer?

The location of the iPad backup files created in iTunes is the following directory:

Windows XP
\Documents and Settings\USERNAME\Application Data\Apple Computer\MobileSync\Backup\

Windows Vista or Windows 7
\Users\USERNAME\AppData\Roaming\Apple Computer\MobileSync\Backup\

Replace USERNAME with your personal account name.

See a list of what iPad data is backed up by iTunes.