The value for the “Amount of disk space to use for temporary offline files” option reverts after you make it larger than 2 GB in Windows XP and in Windows 2000 Professional

via KB904676

CAUSE
This behavior occurs because the client-side cache for temporarily-cached files is limited to 2 GB. This limitation exists because the signed 32-bit integer internally stores the cache size value.

Temporary caching occurs when a client accesses a share that is marked for automatic caching. These files are cached on use. However, the client-side cache may temporarily remove these files if there is not sufficient space in the cache.

This limitation does not apply to manually-cached files. Manual caching occurs when you right-click a file or folder in Windows Explorer, and then you click Make Available Offline