I've had this problem for a very long time. Two years or so. Finally I found a solution(s) based on trial and error on mine and others. The Aspire 5100 indeed has a major hardware flaw that engineers have overlooked and moved on. Rightfully so, due to a steady incline of new technology (AMD Turion 64 x2) and fished as much as they could come out with. Supply and Demand.
Problem #1: Two Loud Beeps during HDD load
This happened without warning. My HDD displayed as: STxxxxxxxxxxx ' ' ' ' ' ' ' ' ' ' ' ' ' '
the only option is F1 Setup & F2 Boot Menu
Solution: I Waited for 10 minutes while in this state, and rebooted (ctrl+alt+del).
The system started up with no problem. Unfortunately this was the method I had to use for awhile untill...
Problem #2: No possibility of boot
Solution: Jam a small stack of three (3) plastic cards (I used Gift cards from any store that sells them. It's free!) on top of the Hard-drive. Screwed the lid back on tightly, and reboot. To my amazement the darn thing worked with no problems or delays.
I've attempted many methods from bending the case to pulling the bastard apart, screw by screw! Nothing.
Others found ways of sodering connections back to getter or reinforcing the motherboard. Few triumphed many failed.
I'm not saying that this is the only FIX, but in my case it worked. I hope this helps.
Currenlty, I'm running Ultimate Ubuntu 2.0 with absolutely no software problems. I'm leaving Vista and XP to the wind.