Strange HD problem

Hi I hope I'm asking this in the right place:
My son's Acer 1640M has just gone belly up.
We had a virus and couldn't get into Vista so I installed Windows 7 at which point everything went well and we looked good to go.
As soon as we got to the input details screen for Windows 7 the computer stopped being able to boot.
The BIOS is accessible but nothing happens after that (the monitor says "input not selected" )

After much googling and trying things out I've reaslised that the pc will boot to Windows 7 boot disc as long as the hard drive isn't connected!????
as soon as i connect the hd again (or a spare) it refuses to boot at all.

Really strange.

I've swapped out components - hd; dvd drive; leads; memory. the only constant is that with hd - nothing; without - boots fine.

Any thoughts as to possible causes would be much appreciated.