Hi,
You need to do a component elimination process to better know what is causing this....
--> Are you getting any beeps from the PC while the fans are spinning...
----> Try this:
1. Open up the side of the case and take out everything from the mainboard (all add-on cards except for graphics card), remove all USB devices, and leave only one hard drive, a PS2 keyboard and PS2 mouse, the CPU and one RAM module connected to the PC... and power up. If the PC successfully boots, then:
2. Re-connect each add-on card (PCI device) and power up after re-connecting each one, when the PC returns back to the state of not booting, then this indicates that the add-on card you just re-connected is the problem...
3. If it won't boot even with only the most basic essential components attached (monitor, CPU, RAM, graphics card, one PS2 keyboard, one PS2 mouse), then this means that either..
-> PSU is fried... (to make sure, test another known working one, if it works, buy another PSU: Enermax, Antech and ThermalTake are good brands...)
-> RAM is fried... (to make sure, test with only one RAM module installed, and if it doesn't work, try different RAM sockets...)
-> Mainboard is fried (maybe you could RMA the board, check to see if it's still under warranty)
-> CPU is fried... (the worst possible case)
-> It would be helpful to know the make and model of your mainboard...