Sounds like some peice of hardware is gone on it.
The new sound beep its making is an alert to help determine which peice of hardware is gone. Find out that make/model your BIOS is and google the number of beeps.
Else, just pull out all your cards apart from the video card and try booting it up then that will let you know that one of the cards you pulled out is faulty.
If it still continues with the error, it maybe your Motherboard, Ram or CPU... or PSU etc..
Windows XP Pro 2002 (SP2)
Intel Northwood 2.4GHz
Compaq Evo D510 SFF
768MB DDR PC2100
NVIDIA TNT2 32MB
Intel 82801 (AC\'97)
C: 40GB (FAT32)