Drive Overlay question

Hi

My motherboard failed so I need to do a new install. The hard drive is from
the computer before last and I want to build the new PC with a new drive but
I want to recover files from the old drive if it is possible.

Can anyone advise if it will be possible, after setting up the new PC with
the new drive, to install the old drive as a slave. The problem is that the
old drive has 4 partitions and required an overlay because my BIOS when I
first installed it couldn't cope with the (then) massive 60Gb.

I don't actually understand how the overlay works, but wonder if the drive
will be readable if it isn't the boot drive.

Any advise would be most appreciated.

Liz