My computer details:

CPU AMD athlon 64 3500+2.2Ghz/512kB, FSB 1000MHz,Soc. 754/939
Motherboard:Gigabyte GA-K8N Pro SLI Soc939/n74/2xPCIe/4xSA II/8xH/1394b/GL
RAM:512 DDRAM 400MHz apacer PC3200, cl2,5,AM1 x2
HDD: 300GB maxtor DM Plus 10 6L300S0 SA/9/7200/16mb
graph card: VGA 256MB HIS Excalibur X800GTO IceQ II Turbo, PCIe,500/1000,256bit,TVout,DVI

So the problem started when i start up my computer today, first it got all the way up to the windows, all looked fine until i tryed to use keyboard, wouldn't do anything, Num Lock light was on but none keys would work, so i unplugged it plugged back, nothing changed, the Num Lock key was green but none keys would work.
So i did a restart but then the monitor went to standby, the monitors light was green and flashing,leaving me with a black screen.There was no beep either.
On the computer the green light was on and all the fans were workin and all, but it wouldn't do anything, even restart wouldnt work, only way to restart it was to hold shut down for 10 secs and then press again, nothing would change, still the black screen.
It was freshly installed WinXp Pro, updated with service pack 2.
Then i just let it stay for a while and then tried again, this time it went black before the windows loading screen.
Did a restart and went black right away like it did earlier, so i just did what i did last time and this time tried to go into BIOS, i got there, but none keys would work, so i couldnt do anything.
I unplugged the keyboard and tried it on this computer and it works fine.
Tried going to safe mode but wouldn't even get to the safe mode option.

Sorry on my bad english, hope you understand my problem, i personally think it could be the hard drive, but im not a specialist so i would preciate your help.

One thing to try would be to unplug the power supply, leave it unplugged for a solid minute, then plug it back in and try to boot, let us know what happens.
Also, when you boot, do you hear any beeps? what is the pattern of the beeps?

I unplugged it for like 10 minutes, it did the one regular computer startup beep.

Then it looked like its going to boot up but just right before the WinXp loading screen it went black, monitor went to standby , the computer still had the green light on, i noticed that the DVD writers light was flashing with after every 1 second thought.
Fans were working.

Now i got another problem, i unplugged the computer off the power for the whole night and today when i came come i plugged it in and tried to power up but all it did it flashed the power light and dvd light for once and then shut down.
Now im thinking about the power supplie aswell.

Do you have another power supply that you can swap with this one? I'm thinking this could be the issue as well.

No i don't but i think i 'm going to but a new one.

Sounds like either a power supply issue or a malware problem. Reinstalling windows is the only thing i know can be done to attempt to stop this issue.