Had this problem on a Windows 2000 client. Caused by (1) Networker Remote Exec Service using Windows Change Journal to keep track of which files have changed since the last backup, rather than using the file archive bit, and (2) Auditing of access to objects turned on at the root directory level (or at least on directories accounting for a large chunk of disk space).
Disable Networker use of Windows change journal and turn off all auditing of object accesses.