System fails to start at all

Have a system with a Gigabyte GA-85661-FXMP-Rz mainboard.
System presented with multiple malware present. Owner had been having probable malware related issues for the past 6 months at least. System is Windows XP Pro SP1.
Many GB of data located on the Desktop.

Initially ran Spybot and Adaware turn about. Kept finding additonal malware.

Found that the 40GB hard drive had only 1GB free space, and XP Defrag showed almost 100% fragmented files ie almost all of the Defrag display was red.

Decided to install new hard drive before attempting to run Defrag etc.
Mirrored original hard drive to new hard drive.
Restarted the computer. Found error indicating low CMOS battery.
Removed battery and tested - 0.47 volts - yes 0.47 volts.
Replaced battery and reset CMOS to Fail Safe defaults

Changed new hard drive jumpers and installed it to IDE channel 1 as master.
Re-powered computer. Processor and case fans activated. System halted with no video at all.
Reset CMOS using jumpers on the board. Same result.
Inspected battery connections and tested battery voltage. Both good.
Re-powered computer. Same result - fans only.

Removed all attached hardware and card. Installed other known good PSU. Installed other known good RAM. Changed monitor to known good. So, ONLY RAM, monitor and power connected. Re-powered computer.

Same result fans only. No video. No POST errors. But ....... after a few seconds of being left with just the fans running, the system speaker emits a rapid series of beeps as in a telephone ring bbbbbrrrrrrrr, but continuous for around ror around 2 to 3 seconds, and then stops. After a short time (30 seconds?) the rapid series of beeps occur again. This is not like any listed POST beep(s).

I have searched for any similar event on the Internet, and have found one which appears to be the same issue involving multiple Gigabyte mainboards where the writer had found the rapid beeps to indicate a failed mainboard.

Of course, this may not be describing the same event as mine at all.
There may or may not be a nexus between the lack of hard drive space, and/or the CMOS battery and/or the malware.
I do believe that my client will not believe for a moment that I have not broken his computer.

Can anyone shed any light on this problem please.