SONY VAIO laptop powers down automatically

Hi

I recently opened my laptop (already void of Warranty) to clean up the dust from the FAN which was giving so much of noise. After replacing the fan - i was able to control the noise - but my laptop starts to powers down automatically. This does not happen always.

I checked the below things
1) Power Supply - no issues
2) Battery - no issues

I am not sure what is causing the laptop to power down automatically. What else should i check to ensure nothing is wrong

Rgds
Vasanth

Is this such an isolated incident? or a stupid question? 90 views but no suggestions?

Did you remove any parts such as the fan, heatsink etc.
Have you left the case open to check to see if the fans spin on startup
You can monitor temperatures with Speedfan.
Could it be windows power management shutting it down?

I removed the fan set - but after cleaning the dust inside - i replaced all the parts just as how it was. I am downloading the Speedfan - how would this help me? what am i looking for? If i start using some software and when suddenly the fan starts to run fast and later the system switches off

Thanks !
I am trying to understand that tool right now.

How do i control the fan speed ? Is there any way i could change it so that the system does not switch off? It does not happen after prolonged use and all - system shuts down even just one program is active in the system