Which component causes the system(WinXP Pro) restarting during playing games(C&C Generals and SplinterCell)?
CPU: AMD XP 2100+
Memory: Kingston DDR333 PC2700 512MB
Motherboard: MSI KT3 Ultra ARU
Vedio Card: Leadtek WinFast A280TD
SoundCard: Creative Audigy2
Harddrive: WD 120GB
Power: Turbolink CWT-420ATX
Thanks for help. How big power supply should fit my system?
I'd say that probably any supply of the ATX standard should fit your system, unless you have a MicroATX case or something...
Seriously though, as idealistic as rating power supplies by watts is, it is also about as meaningful as rating processors by MHz. Size is not the key issue. More does not actually equate to better. So I can't really just give you a one-size-fits-all answer.
If you can monitor the rails of your power supply (3.3V, 5V, 12V) and tell me what voltages these are at (specifically before a reboot) then this would help tremendously for determining if it is in fact the power supply.
Should I need to buy a UPS to present the stable currency for my system?
You shouldn't <i>need</i> to buy a UPS. However, I don't know the state of the electrical wiring in your house or even your town. It is theoretically possible that if you have bad wiring your problems might not be with your PC at all and a good UPS could indeed fix the problem. If you have good wiring though, this isn't likely to be a concern.
Another thought is what other things are plugged into the same circuit that the PC is plugged in to? I have a friend who told me once of a laboratory that he worked for was having their PC crash regularly. They put a thermometer to record the temperatures of the room over the weekend to see if it was a heating problem and found that the computer kept crashing moments before the room temperature dropped slightly. In the end they figured out that the problem was that the air conditioner in the lab was causing a power surge that would crash the PC. Heh heh.