First thing is that your power supply is marginal for your hardware. Go here PSU Calculator
and put you computer hardware into the PSU calculator and you will find you need at least 425 watts. Most PS's run at 70 % efficiency so the real rating of your PS is about 300 watts. The numbers stated on the PS's are usually peak numbers.
Do the fans in the computer turn on when you hit the switch? Any lights come on? cdrom, dvd drive, power lights? If not i would suspect the power supply is not working.
I just built a intel celeron puter for someone. I bought a kit from tiger direct and the first thing i did was check to see if it would power up. I did not have any cdrom, dvd, case fans, or sound card hooked up. All i had was MB sitting on a box, cpu & fan, 1 stick of memory, and a HD. The puter has onboard video. The computer booted to the intel startup screen and would go no further. I installed everything in a case along with the other hardware and stuck a XP cd in the cdrom drive. I still could not get past the intel screen and i heard a clicking sound. After checking memory i replaced the hard drive and windows XP booted up and started installing.
The point here is it may be the hard drive causing the problem. But you still should be able to get to the bios startup screen. That makes me think its the PSU.