Fans on full blast usually means that the board hasn't finished its POST sequence, and something is holding up the successful Boot of the system. To completely isolate this problem, I suggest the following:
1)take the PC completely apart, including removing the motherboard (but leave the CPU & RAM on the board)
2)Place motherboard on a flat, non conductive surface. If you still have the box the motherboard came in (if you built it) place the board on top of that box.
3)If not a self-build, put the board on top of a cardboard box of the right size, then,
4)Plug in the power supply to the right ports on the board, video card, etc. Do NOT connect the hard drive, CD/DVDRW drive or anything else at this point. Just the power to the mainboard
5)Plug in your video card, connect the monitor, ensure that the video card has it's power cord as well, if required.
6)Find the pins to turn on the system on the motherboard. Short them out with the end of a screwdriver (it's safe) to get to PC to turn on. Observe if the system starts up, or if the fans on the CPU/GPU just spin at full speed as they did inside the case
6a) If the fans spin at a really fast speed, unplug the power from the power supply and disconnect the PSU from the motherboard & video card
6b) Pull the CMOS battery from the motherboard and wait 10 minutes (5 is usually enough, 10 to be safe, your call)
6c) Leave the battery out, reconnect the power supply to the board, video card and attempt to turn on
6d) Does the system POST? Do you get any video? If not, issue is related to either the RAM, motherboard or CPU. Video card is a possibility at this point, but I doubt it
7) If the system DOES turn on, shut it off and add one component back to the system at a time (Hard drive, power on, if it works, shut it down and move on to the CDRW/DVDRW drive) Ignore any errors about the CMOS battery or BIOS date/time being incorrect, just continue
8) Once you get all of the components reconnected to the system, and it continues to turn on without issue, power it down and re-insert the CMOS battery
9) Turn the PC on again and save the BIOS settings (if it kept prompting you on startup in the prior steps that the BIOS wasn't correct)
10) Verify OS load (windows, etc). If you can get back into Windows, shut the PC down and re-build it inside the case, being careful to repeat the same power on / check cycle as you rebuild the system, so that if it happens again, you can identify which component is giving you issues.
11) If no issues upon successful rebuild, then celebrate. If not, find out what is shorting out or giving you issues. If you didn't get this far, and had to stop on the "6" step series, isolate the bad part and buy a replacement or ship the offending part off to RMA if still under warranty.
