As chris.stout indicated, these beeps mean something is wrong..obviously. The advantage to the beep codes is that you do not have to have a monitor to determine what the problem is. Depending on hte patern of the beeps, you should be able to determine the issue.
I have this exact same desktop, and had this exact same problem
the way I fixed it, is to replace the graphics card.
The beep normally means that the desktop cannot detect a graphics output so it just powers down and makes one long beep