Keep settings and managing disk space during recording in MCE 2005:-

1]Until Space Needed:-If there is no available disk space, the recording is marked as available to be deleted to make room for any new recordings. The show is deleted when more room is needed.

2]Until I Delete:-The recording is kept until you choose to delete it. Too many recordings marked Keep Until I Delete could fill up available disk space and cause future shows to not be recorded.

3]Until I Watch:-The recording is kept until you watch it. Once watched, it is kept until space is needed for new recordings. Too many unwatched shows marked Keep Until I Watch could fill up available disk space and cause future recordings to not be recorded.

4]For 1 Week:-The recording stays on the hard disk for one week, even after you have watched it. After a week is up, the recording is kept until space is needed for new recordings.

5]Latest Recordings (Series Only):-Media Center records the most recently broadcast episodes of a series, deleting older recordings of the same series (whether you have watched them or not). The number of recordings kept for each series is specified by the Keep Up To setting.

[source:Microsoft]