sunima_sh,
did you check the CPU/motherboard temperatures? Install the EVEREST tool as I mentioned above. Check if it exceeds the maximum CPU temperature that your CPU can withstand. Check one my posts above for more details. Make sure your heat outlet is not blocked.

Which model of Acer do you have? You can get the details from : Control Panel -> System -> General tab (XP classic view); Control Panel -> System and maintenance -> System -> General tab (Visa & XP category view).

blue_moon,
unchecking the 'automatically restart' will NOT solve your problem. This option is there when your system fails, otherwise it will simply shutdown. sunima_sh, do you have this option unchecked? It may be the reason that your system is simply shutting down without a restart due to system failure!! I do not recommend unchecking this option.

Most probably you have 'Spyware' problem, as most of the people these days have. This is the most common problem these days. To check if you have a spyware/adware, go to:
www.pcpitstop.com
click on full tests -> new members -> create an account (you dont have to) -> Let's go -> OK
When the results are displayed, click on "Test Details" on right under "Current Test Results". In the "Running Programs", check if any program is listed as spyware/adware.  You need to remove them. They collect your personal information like shopping habits, what you search on internet, activities. They run on your computer all the time, hence making your system slow.