Reboots Every Hour

i have had windows 7 64bit version installed for a couple months on my pc, and these 2 days it's kept rebooting. it goes off (no shut down message, it all happens all of a sudden), and after a second or 2 the machine boots up again. before loading the os the pc prompts me to choose if i want to start windows normally. i press enter and windows starts up with no problems at all. when a system error occurs, after rebooting it usually notifies me that the system was recovered from an error. but it displays no message.

now, my question is this. i heard that before the coming out of the windows seven final version, microsoft would have you install an update that reboots your machine every 2 hrs to "suggest" people buying the os... how kind of them, really... whatever, is this possibly the case? if this is, i guess i gonna downgrade to windows vista till i can buy windows 7.