At POST time (when the machine is first turned on) when nothing happens but the machine just keeps beeping (Please note this is not the same for Laptops). These may be different for your machine but I try to point out a possible problem!!! Those listed are the most common ones I have come across in my time repairing computers.

No tone (nothing) - Possible power supply problems, power cables or Motherboard couplings on wrong way round (AT connectors have black wires that MUST go together onto the connector)

Continous tone - Power supply defective, this needs replacing

Many short beeps - Defective motherboard, could be a connection loose or ill fitted items.

1 long beep - RAM refresh error, try taking some RAM out and replacing it (72Pin modules need to go in pairs in most Pentium boards)

1 long, 1 short - Again defective motherboard or the ROM-BASIC part of the BIOS. A motherboard replacement is in order.

1 long, 2 short - Video card error (or on XT's dip switched), try another video card or check its seated correctly and all cables are firmly plugged in.

1 long, 3 short - Video card error (most caused by monitor cable not plugged into the video card properly). If the monitor is attached correctly then try another video card.

2 long, 1 short - Video card error (see above)

2 short - Parity error, check memory installed in system. You could try checking the CMOS for a Parity option and try Disabling it.

3 short - First 64K of RAM error, check to see the RAM is fitted correctly, if so try changing it.

4 short - Real Time Clock problem, only fixed by changing the RTC (if possible) or changing the motherboard.

5 short - Processor (CPU) failure or video RAM check the video card is fitted correctly and the processor (try changing the video card BEFORE changing the CPU if needed).

6 short - Keyboard error, check the keyboard is correctly fitted, if so change keyboard.

7 short - Virtual processor mode set (Factory set mode to test systems), try changing processor or motherboard.

8 short - Again another video RAM problem (see above)

9 short - Incorrect ROM-BIOS checksum, try flashing the BIOS or changing the BIOS chip. If you have already tried that change the motherboard.