Don't know why this happens, but I have had it also... but try going into administration tool.... services look for what reflect restore... start it as automatic... and see if that works for you. It worked for me... but occationally have to do it again and again..
I have a new laptop that came with Vista and every time I reboot my computer and right click on My Computer and go to the System Protection tab I am told that I have no restore points. I create several of them then reboot and go back in and they are missing again. I haven't turned anything off except Defender and the Windows Firewall because Trendmicro does it automatically and keeps them turned off when you install it. I am afraid to install driver updates because I may have to do a full hard drive reinstall...Already had to do it once when I installed Symantec pcAnywhere (crashed to blue screen 10 times even after using the recovery procedures through Vista and kept BSOD-ing). Did a Vista Forums search on Google on this problem and so few people have forum responses except on CNET and everyone has problems with XP/Vista dual boots, but that is not the case here. Any suggestions?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic