Power Up Failure [URGENT]

Help!!! I've got an urgent problem about my desktop computer at home!

I had a PIII-450MHz (Slot 1) computer with 256MB SDRAM and PC100 Systemboard M741LMRT. Originally it is using a small metal case with only 1 fan space available. This results a high internal temperature. THerefore I changed the case to a bigger one, and at the same time replaced the power supply with a DELUXE EWP-3300A(ATX 12V & 2.03).

However after connecting all the wires and ensured that they are tight, the system even fails to display any screen. This is what happen when I press the Power button:

1. Power On.
2. Power LED on.
3. HDD LED on.
4. CD-ROMs Lights on.
5. CD-ROMs Lights off.
6. HDD LED off.

Meanwhile the screen remains blank and an orange light is blinking on the monitor Power LED.

I had unplugged the only PCI card I have - an IDE card - when I retry. Things just remained as they were.

I had checked that all the harddisks and CD-ROMs are running well, and the CPU, RAM, on-board AGP Chip, on-board LAN chip and on-board Audio Chip are all warm, indicating that they are working.

I just can't figure out what is happening to my system, and I need to fix the problem ASAP. Do anybody have any ideas???