>This has happened on my Vista HP system several times.
Anyone know why this might happen?
(I haven't run any disk clean-up processes.)<
You are using a dual-boot with WIn XP? If yes, this is a Windows XP
feature.
If not, some third party defragmenation tools, virus scanners and so on are
causing a bunch of shadow copies - as a result, the free storages exhaused
and old restore points are deleted. Details are discussed here
http://gborn.blogger.de/stories/790875/ (but unfortunately in German only).
Maybe it helps.
G. Born