Event Viewer - "unexpected Shutdowns"

My 'unexpected shutdowns' are random and occur -on average- once every session. They vary but usually go by two main scenarios: (1) Vista desktop and mouse cursor freezes - control/alt/delete and keyboard too - the only alternative is to push the PC restart button - or (2) the monitor goes black and Vista reboots on its own. What's usually happening when these scenarios occur? Well, I ought to keep a log ready to record these events. I'll soon have to try but as a student in a much earlier life - I took such bad lecture notes I could not read them after two class lectures. Here's what I can recall:1. I think most shutdowns have happened when either beginning to view a video or going to a new web page that pops up a streaming advertisement.

2. I recall one when I opened Windows Mail - but cannot say if I had clicked on anything yet or if mail was downloading at the time. So - it may not be Windows Mail but just the action of opening a new window? My Internet research indicates these 'unexpected shutdowns' have been frustrating Vista & earlier MS OS users for years. Rather surprisingly, I found no promising suggestions for troubleshooting or remedies - to date. For years the OS of my choice was W2K. If only Vista had W2K's reliability. I rarely - if ever- experienced such ongoing, unexpained W2K events.Perhaps I've missed some new info that addresses this issue - so that's the primary reason for this posting. Anything? I should add that upon rebooting after an unexpected shutdown - Vista asks if Safe Mode is desired during rebooting. I never benefited by going to Safe Mode - except that on the many occasions that I bypassed Safe Mode & choose normal rebooting - I would experience a variety of other issues soon afterwards. So - Safe Mode is probably the best way to go if one's patience permits - even if it avails few if any hints as to the specific shutdown