Disappearing System Restore Point

I have scanned all of the MS Vista news groups and have still the following
problem.

I can create a System Restore point, either at request or automatically at
start-up. The issue is that it disappears at next boot.

- My system is an update of XP Pro to Vista Business - but no dual boot;
- It runs Diskeeper and OneCare Live. The problem persists if I disable
Diskeeper on the Operating System (OS) disk;
- Performance score of the disks is 5.9 (2 scsi drives, one of 80GB (OS) and
one of 160 GB - not in an array nor in raid)
- I have increased the allocated space for restore points to 10GB, using
vssadmin
- I have delayed the start-up of the System Restore Service to 30 minutes
after sign-on (to avoid Volsnap 25/27 issues)

All hints are welcome.