My compac lapton using vista shuts down

Hello,

My compac laptop using vista shuts down after about 20 minutes of starting it ,Right before it shuts down it starts to get louder then normal ,,kinda like its running faster.It doesn't seem to matter what I'm doing ,writing a email or just surfing Geocaching.com .After it shut down if I try to turn it on it will shut down again ..I can wait some a little while and it will start and run again for a few minutes and do the same thing over again ,when it boots it always ask what mode do I want to start it in,I think the fan is running because it sound like whirling noise coming from the laptop,,Thank you