PC keeps rebooting after a few seconds.

I'm not sure what it is it is. The problem is when I turn my PC on, after a few seconds there would be a beep and then it turns off. It would try to start again but it'd just beep and shutdown after a couple of seconds. The process would continue unless I hold the power button or turn the switch off on the PSU. It can't even get to POST screen.

I'm not sure if this is related, though earlier today my PC would randomly shutdown and restart when I was doing anything even as simple as watching a video on YouTube. It did it at least 30 minutes after I turn my PC on. One time my monitor couldn't get any signal but it did get signal a few minutes later. The last time I was able to do anything on my PC, I was playing a game. All of this happened today.

I asked someplace else and apparently my HDD may be overheating?

Core2Duo E6550 2.33Ghz
Intel DQ35JO
2 x 1Gb Samsung 2Rx8 PC2-5300U
Seagate Barracuda 7200.7 80 GB HD
ISO-400PP 300w PSU
GeForce 7300 GS

Your 2 GB memory need more power, 300 watt is not enough, get more power..!
430 watt or if you brave enough 500 watt..