Power Supply & hanging problem

I have been having strange problems with my desktop computer and I need help again.

(i) At times on starting the computer the processor fan runs but there is no display on the monitor. I had to force shut the computer by holding down the start button for some time. When that happened for the first time I thought that the monitor cable was loose at the PS2 connection. Fiddling with the connection a couple of times the monitor came on.
(ii) When the same problem reappeared again I was unable to get the monitor to work even by tightening & fiddling with the PS2 connection several times. In order to eliminate the problem in the monitor cable I attached my laptop PS2 to the monitor & the monitor was found to be in order raising doubts about the PS2 port on the computer or the Motherboard.
(iii) When the monitor went off again I removed a cable from the Hard Drive to reduce the load on the Power Supply and restarted the computer and the monitor came on. I force shut the computer & re-attached the cable of the hard drive & the monitor came on again & the computer booted up normally.
(iV) After the computer came on I found that it had developed a horrible hanging problem. Cont+Alt+Del & the mouse were not working and the hour glass was perpetually on. After force shutting the computer I re-started it on Safe Mode and checked my computer for viruses & malwares by applying Malawarebytes, AVG, Ad-Aware and Trojan remover. None showed the computer to be infected. However I observed that the computer had become exceedingly slow.
(V) During the course of all the above problems I found the the system clock had failed and it was showing a date in the year 2002.

While the computer continues to run in the same manner with the monitor coming on erratically and the computer running extremely slow I am unable to diagnose whether the problem lies with the Hard Drive, the system clock battery, the Motherboard or the Power Supply Unit.

The Intel Motherboard is D865GBF/D865GLC and my Operating System is Windows XP SP3. Any help in diagnosing my problem will be highly appreciated.

Very probably the BIOS battery is old, you need to change it.
http://i.imgur.com/gGMqF.gif
They're standard, go to a PC shop with it and get a new one.