If you bought the 40G HDD new, it wont have a partiton on it, therefore wont show up as a drive in windows.
What you need to do is add the partition and foramt the drive, so that is will be displayed in windows.
WARNING: Do this at your own Risk. Make sure that you backup any important Data incase you make a mistake.
You can do this by putting in the Windows 98 Start-up Floppy Disk.
at the A:/>
type FDISK
there should be an option 5, chnage current Disk...
in there it should show 2 drives
1 which should show ur 120G hdd
and 2 which should show ur 160G
Select Number 2, the 160G HDD
Now go to Opinion 1, to Create a Partition, then Option 1 to Create a Primary Partition. Select Yes to the options that follow. Once that is done press ESC from the main menu and restart the comptuer, booting back to the Floppy Disk.
Now at the A:/>
type FORMAT D: /u
This will then format the 40G to make it readable.
It will say, are you sure you want to format, Yes.
Take the floppy disk out, restart, and then it should be under My Comptuer as D: