I recently attempted upgrading my hp p665w computer. It had all stock parts save for a 300w psu and an old, but better than the built in graphics, card from a friend. I replaced the default cpu with an AMD Phenom II black edition cpu, the old graphics card with a Geforce GTX 650Ti, and the power supply with a 550w XFX psu. All seemed to be going well as I replaced these parts. I took care not to move the computer while working on it, I made sure to discharge any static I may have had as well as grounding myself during the process. Upon plugging the computer back up though I noticed that it would not stay on. I counted 7 seconds repeatedly as its time when it shuts off. I opened the case back up to check the motherboard for any touching metal as well as unplug and replug the psu wires to check for good connections. I still had the same problem though. It appears nothing is wrong with it aside from it not staying on. The motherboard lights up green with its LED, the machine beep as it turns on like usual, all the fans spin and I cleaned the only dusty one, but still the computer fails to stay on. I would really appreciate some help with this if anyone knows what could be causing such an issue.