Does it beep just once? Does the computer have any issues or problems you are aware of?
If its just once and there aren't any issues.
Nothing wrong with it. That beep on start-up means the motherboard has POST'ed, which basically means that its booted into the BIOS. That beep is actually so you can tell the motherboard is working if you don't have a screen attached to see the POST display.
If the beep is annoying you, pop open the case and remove the motherboard speaker.
Nothing comes to mind.
If the machine works fine, I don't think there's a problem. The motherboard beep system only tells of catastrophic failures (like bad RAM, bad CPU, missing power cable and such).
Check your motherboard manual, it should tell you what combinations of beeps mean.
So just look at what the PC displays during startup (before the Windows logo), and see if anything along the lines of an error shows itself.
Nope, that was old IBM BIOS. American Megatrends BIOS is what most motherboards use. Then it indicates a Parity Circuit Failure, which if this website is to be believed, has something to do with the RAM. http://www.pcguide.com/ts/x/sys/beep/amiB2-c.html
Check in Windows if the full 4GB of RAM is detected (go to "Computer", should say at the bottom). If it is, no idea whats wrong then.