Why my computer always shuts down without warning?

[Sangmo]

If you mean it just suddenly turns right off, or suddenly reboots, without any warning, then it could be a hardware problem. Maybe the computer has become very dusty/dirty inside - which most do over time - and this is causing problems for cooling fans, or even causing electrical leakage. What I would do is give it a very thorough clean up, taking out the RAM and all add-on cards to clean them and to clean the sockets on the motherboard that they fit into. Be gentle but thorough, take your time, and make sure you are earthed to minimise the risk of static electricity. Don't remove the CPU fan or CPU unless you are familiar with what is involved. I use the smallest brush on a vacuum cleaner to suck away dust (very slightly moistened to reduce the risk of static), and an aerosol can of compressed air to blow dust out of any little crevices and sockets. After the clean-up, when everything is reinstalled, check that all fans are working properly. If this doesn't solve the problem, it is possible that there is some sort of failure developing in one of the electronic components. It may be possible to work out which component is failing by trying to use the computer without non-essential parts like communications cards, or with substitute parts. If there are two RAM modules, try using it with just one, each in turn. If you do this systematically, one part at a time, you may find the particular part that is causing the problem, and it will probably be very inexpensive to replace. Only if the problem is with the CPU or motherboard will it be a bit more tricky to deal with. But there's a good chance you will be able to sort out the problem relatively simply by doing what I've outlined. It's good to keep those old PCs going. Best of luck!