No problems . Take the side of your PC case , disconnect the IDE cable to your existing drive , disconnect the power cable too .Take the drive from out of the external case and link this 40 gig to the IDE cable on your PC , link the power cable too . Boot up , go into the BIOS ,change the first boot to floppy . Get a boot disk from bootdiskdotcom first and start up the PC .Use the fdisk command to delete existing partitions , then create partition and finally format . Swop over the drives and put the cases together.
This is by no means a complicated business although it may look it. It really only takes minutes to do. I have five external drives ,but they were empty enclosures when I bought them , you just add your own hard drives and this is what I did for them . You should really have a boot disk in any case , sometimes they come in handy.