OK... Either your PC uses CS (Cable Select) or MS (Master/Slave) designation to ID drives found on the internal cablesets. Find yours for the 160gb here: http://www.wdc.com/en/images/products/drives/10_pin_jumper.gif
OK, you can't mix types, and you cant put 2 Masters or 2 Slaves on the same cable. Soooooooo
You need to find out what the current HDD uses... so pull the current HDD out, turn it so that you are looking at the backside (where the cable and power plugs in) and look for a group of 6, 8 or 10 tiny pins in a row ( Jumpers)
Look where the Plastic pin(s) are if any and then look on the BACK, Top, PCB and Rear cover for the diagram that explains the position of each.
If your current Drive is CS, then just set your new drive to CS and plug it in (on the same cable)
If your current drive is all alone on the cable and there is a free connector hanging loose, it is most likely set to SINGLE MASTER, and will have to be changed to DUAL MASTER... then set your new drive on DUAL SLAVE and plug it in (on the same cable)
If your current drive is NOT all alone on the cable and there is CDROM attached to it too, it is most likely set to SINGLE MASTER, andyou will leave it alone and you will set your new HDD to SINGLE MASTER as well, and put it on the OTHER cable.
You may have to alter your BIOS settings to see the new 160Gb HDD IF your motherboard is capable of handling that size. Press F2 or DEL or Cntl-S or F12 during boot to get there, then make your changes, save and get out
Once you can SEE both HDD in your system, then you either use a program like GHOST and do a bit-by-bit copy of the old HDD to the NEW, and then make IT MASTER and the OLD one SLAVE (and reset BIOS settings to reflect this) or just leave it in there as 2nd BIG HDD with lots of space.
Find yours for the 160gb here: http://www.wdc.com/en/images/products/drives/10_pin_jumper.gif