Your system has a Phoenix BIOS which has very complex beep codes. They either have a series of three or four beep codes. For example, a beep code combination of 1-3-4 (one beep, pause, three beeps, pause, four beeps) means that the motherboard is bad. You can see a full listing of the beep codes for Phoenix BIOS
here. You say that it's one long continuous beep, but this doesn't make sense for your system. See if you can discern a pattern of beeps.
My guess is that your problem is either with your video card, RAM, motherboard, or power supply. How's that for a vague answer? If possible, see if you can get your hands on a friend's AGP video card and power supply to try in your system. This way, you can at least rule out those components.