Computer restart without warning

hi guys,
basically what i want to ask about is, what the hell is going on my pc?
i overclocked a few weeks ago and the IBT (intel burn test) was successful so i'm not sure what's going on, or even if it has something to do with IBT, but it's really annoying and frightening for a few reasons:
1. it happens without a warning (no noise, nothing overheats, no over-usage, no nothing).
2. it does not save my progress
3. there's always the fear i damaged my pc.
i don't know why this is happening. it sometimes even sends me to blue-screen but immediately restart again. also is there any chance it might be ram related? i'm having some problems with my ram also.
specs:
fx-4100 OC'd to 4.2
4GB ram (pretty old)
6850 1GB
550 watts psu
thanks everybody!!