Problem with power to PC

Hi,

I have over the last few months had problems powering up my desktop PC (Dell Dimension 4600).

First problem:

I turn the plug on but the PC doesn't turn on (even if I press the start button on the front of it). If however I leave it plugged on for about 10 minutes, the PC eventually starts up. Any idea what could be causing this? This problem only appears to occur if the PC has been unused for a while (say over an hour) - if I've recently used the PC it does power up ok.

 Second problem:

When the PC does finally decide to turn on, the power light comes on for a couple of seconds and the hard drive makes a noise, then it turns off again, then it comes on again for a couple of seconds and keeps doing this on-off routine until I press the power button on front of the PC which causes it to boot ok. Any ideas what could be causing this?

I recently had a problem with the clock not setting the time correctly and continually reverting back to 2003, I replaced the PC internal battery and it's now ok - not sure if the current problems have anything to do with this?

Thanks

Simon