Than you all for this very helpful thread. My laptop (Dell lattitude D630) was starting to act up like for the last few months and it was still quite ok. It would shut down only sporadically, but for the last three days it wont stay on for more than 10 - 15 minutes and I was going nuts about it. It is my work computer.
After researching in the internet, I cleaned the fan, opened it up and cleaned it well with a canned air, but nothing was able to help. This morning I came across this thread and after going through it, I did few things which seem to fix the problem (hopefully!!).
First, I removed the RAM and cleaned it and put it back. Then I installed CC Cleaner and cleaned the registry and unwated files etc, few gigs in total and finally I upgraded my windows xp and the laptop seems ot be working for few hours now without shutting it down. Hopefully I fixed the problem and hope this information help others in need.