Losing powerrrr....

Well, my problem started with my PC shutting down for no apparent reason. I would restart it and it would run without problem. But, this became a progressive thing where it would shut down more often. It finally reached critical mass when I tried to do a Norton Defrag. It would shut down during the process and then after a restart or two it wouldn't even power up again. One of my 'cures' was to switch the power cord to another outlet thinking it was the power strip's fault. But, I figured out that it was the PC. I'd unplug it from the back of the PC, replug it in several times which usually got the power light on the back to come on. It usually took blowing in the plug (moisture helping?) to make that happen. When I pushed the power on button it would try to start for a second or two and then nothing. I would repeat the replugging step and same results. I imagine it's a power supply issue, but, if anyone can give me any advice on a cure of the problem beyond taking it to the repair shop I'd appreciate it. I'm trying to do it economically and on my own if possible. Thanks for any help! Oh, btw, this may or may not be related, but, when I do a Norton Anti-virus scan it won't scan from a particular folder on my start up menu. It gives an error message. If I scan the rest of the PC it comes back without any viruses. I'm pretty vigilant with my firewall and deleting unknown email messages. Thanks again!