According to a general beep code chart that I got from a Dell support person, 5 beeps is "Realtime Clock Power Fail" and  possible cause is CMOS battery failure.  

I know this is years after the original post, but I'm posting this because here's how I got it to work for me, and it might help someone else searching for a solution.

I inherited my D540 from a family member who maxed it out with four 2GB DIMMs for a total of 8GB. Here's what I did when the computer wouldn't turn on:

1. I'd changed out the power supply for a cheapo DiabloTek, then got a 450W Antec.
2. I also took out the old CMOS battery, waited 30 minutes, then inserted the a new battery  (CR2032 button cell, about $5 at a drugstore).
3. I also drained the flea power in the system by unplugging the power supply, then holding the power button for at least 20 seconds.

I figured I was golden, and turned on the computer. The power light turned on, I could hear the fans running, but then I heard a series of five short beeps. Nothing on the screen.

In another thread somewhere on this site, I saw the suggestion to take out all DIMMs, then seat them only ONE AT A TIME in the slot closest to the CPU, then turn on the machine. This is to see which memory module, if any, was bad.

When I did this with the first DIMM, I still got nothing on the screen and the five beeps. I took out that first DIMM and put it aside.

When I put in the second DIMM and turned on the computer, voila! The computer turned on, the display lit up, and NO BEEPS! The BIOS screen told me there was a clock error, so I pressed F2 to set the correct date and time. I saved the changes, and the computer rebooted.

The computer loaded normally. It did take a while to load Windows, since there was only 2GB in the system. After that, I shut down the machine, turned it off, and put all the memory back in--even the "bad" one, just to see what would happen--and I could see all 8GB again when I booted back into Windows.

I hope this helps!