|"The BIOS does not recognise the new drive - probably because it is 5400rpm rather than 4200."|
I searched the web using: Aspire 1640 hard drive upgrade
and there are lots of "hits" for hard drives larger than 137gb, so your bios does NOT have a drive size limitation.
The rpm of the drive is not something the bios can detect, and has no bearing on the matter.
Did you install the new drive inside the latop, or do you have it in a external enclosure and the computer isn't recognizing it ??.
If you installed the new drive inside the laptop...
Did you REMOVE the main battery as well as the AC adapter's connection when you removed the old hard drive and installed the new one?
Check your bios Setup and make sure the drive detection is set to Auto by the method Auto or LBA - if the bios is set to specific parameters for the 40gb drive, the 160gb drive will probably not be detected.
There are 4 pins on the header on the hard drive that don't plug into the connector to the laptop. You do not need to install a jumper on a pair of those pins, unless you can install two hard drives inside the laptop and you will have two installed when you install this one. Usually you can't plug the laptop connector into the pins the wrong way, but check the User's or Owners manual for your model to make sure you have plugged the connector into the right pins on the hard drive.
Manual for your hard drive:
If you have it in an external enclosure....
It may not show up in the bios Setup.
It should show up in Windows...
USB devices may not work correctly when they're connected to certain USB ports. Instructions for how to re-load your USB stack in 2000 or XP, if you need to - similar applies to Vista and Windows 7.
In addition, all external drives connected by USB require that the port (or ports) they connect to can actually supply 500ma of current.
USB external hard drive requirements.
See Response 9: