(Redirected) Desktop PC Issue(s)

My 'Dell OptiPlex 745' shuts down automatically and then tries to start again and again but in vain. It has started happening about three weeks ago. After one week of the problem, I took it to the shop from where I purchased it. They replaced its power supplying box that resolved the problem for hardly 1 week. Now the issue is there again. What do I do. Please help. My computer system is running on 'Windows 7'.