I don't understand what you've written.
What I guess that you mean is that your old 40 GB HD is not found by fdisk
running from a DOS boot floppy. That would mean that the old drive is dead.
You can still buy 40 GB drives:
http://www.newegg.com/Product/Product.asp?Item=N82E16822140118
but you can buy a 120 GB drive for not much more money:
http://www.newegg.com/Product/Product.asp?Item=N82E16822148104
The current sweet spot in pricing might be 250 GB:
http://www.newegg.com/Product/Product.asp?Item=N82E16822148062
(about $100, or 6X the size of the 40 GB, for a bit over twice the price.)
Some motherboards have trouble with hard drives that need LBA48 addressing.
(Those drives are larger than 137 GB.) Some of those motherboards have BIOS
updates available that will fix that. Older boards have trouble with drives
smaller than that, but I wouldn't expect to see that with any P4 boards.
If you could post what your motherboard is, someone might be able to tell
you what size drives it supports. There are utilities (SiSoft Sandra is one)
that can ID a board, but they require a working PC.
Hope this helps.
Address scrambled. Replace nkbob with bobkn.