repairing my computer

I brought my desktop computer with Windows Vista from Japan, but maybe for some electrical or programming problems, my computer doesn't work well. I can use this computer steadily but, as about 20 minutes pass by, it turned off suddenly. I really need someone who is good at repairing a computer. If you succeed in repairing, I will pay $40. Please give me your help. I gave you every information about this computer if you want to know. Thank you.