Vista - Start Menu Items Dissapearing

Have been using Vista Home Premium 32 for about a couple of months. Noticed in the last few days that when i click on the start button and then all programs, that a bunch of the program items are gone. Most of the windows related items, like Windows Calender, Windows Defender, Windows DVD Maker, Windows Media Center, Windows Movie Maker, Windows Photo Gallery and other items, like HP total care advisor, that were there last week are no longer there. I can use exlorere and locate them and add/pin them back, but why were they removed? TIA.