I have bought a new desktop machine with AMD phenom Processor and JM26GT4 motherboard.

My system used to shut down abruptly without giving any message, i used to work in Windows XP, recently i have changed the operating software to Windows Vista (Basic Home edition), it worked fine for first 2 days and now again it has developed a habit of getting shut down as if somebody is putting off the power.

In system maintenance log i found "System Service error", which stopped while installing automatic updates of windows.

i dont know how to solve the problem, whether it is a software problem or any hardware problem.

that is a symptom of CPU Over Heating...get a free program or something to monitor your CPU Temperature...if it's over 50/60-80C+, than it's CPU over heating...but it would be odd since it's a new machine...but either way, check it to make sure...

I have tested the heat sink of the CPU it is perfectly connected, in the BIOS setting the CPU tempretaure is set to be monitored "auto".
Its getting turn off sometimes within an hour, and sometimes in 3 hours, while quite often befor starting windows it turns off.