It could basically be a temperature problem: being a long time on > temperature rises > crash. Another fact is, if you turn it off, it can create a sort of "after heat", since fans are turned off > creating an over heat effect when you turn it on again; this is why some users have "post cooling" i.e. their fans keep running for say, 15 minutes after shutdown.
Honestly, all I can say is that you should give your computer a proper vacuum cleaner treatment. Just puff all the dust off! The reason it began after you went over to Windows 7 could well be that Windows 7 required more power = more electricity = more heat. And any dust will raise creation of heat.
The fact that your computer hasn't broken down shows clearly, the components are functioning.
Whilst waiting for proper guys to give an answer, I suggest you give it a "dust treatment".