computer is shutting down with system overheat error

Assuming you've only changed the fans, and it is still reporting an overheating issue, have you cleaned the heatsink of any dust, etc?  Fans will only do so much, but if the heatsink is full of dust, there is a reduction in the amount of heat dissipation that occurs.

Also, not knowing how advanced your system is (monitoring, etc.) the power supply could also be sensing a heat issue, and may also need to be cleaned.

yes I replaced the heat heat too.

If you are getting an over heat error as mentioned in the title... even after replacing the fans then like previously mentioned the heat error could be caused by the CPU sensor, which may be a problem with either inadequate contact between the CPU and heatsink, or a dirty heatsink. A thermal event error could also indicate that there is too much dust and dirt on the motherboard as well. Try blowing out the case with a can of compressed air to see if that helps. While you are  at it look on the mother board to inspect the capacitors on the board. The capacitors are the little tin can shapped components on the board. They should be flat across the top, not bulged on the top. They should be perpendicular to the board (straight up and down). There should be no brown or white crusty stuff either on the board or on top of the capacitors. If you see any abnormal capacitors this could be the route of your problem. See more information on how to recognize bad capacitors go to http://en.wikipedia.org/wiki/Capacitor_plague or http://www.badcaps.net/

When you replaced the heatsink did you clean off the old paste or stuff from the cpu and use adequate cpu grease. You first need to thoroughly clean the cpu from the previous heat sink compound, using a cleaner such as isoproyl alcohol.

yes, I air dust the system completely and I did clean the cpu when replacing the heat sink. I'll swap out the power supply and get back to you guys...thanks.

Try to update the bios to the most recent version, sometimes that may help. Check the manufacturers website for your motherboard, to see if there is a more recent bios version. if the powersupply does not solve the issue. Did you check the capacitors on the motherboard to see if they appear faulty?

since you don't say what pc you have - we can only guess.
a common cause on older pc's, are bad capacitors on the mobo - or PS.
check for bulging or leaking ones.
the parts have to be replaced, or the mobo / PS

**but it really can be anything.  the best way is disconnecting or replacing devices to test, and keep a list

i've replaced the power supply. I'll keep an eye on it for couple of days.

i replaced the power supply and the problem was gone.