check the motherboards on both machines + check the dimensions of the HDD model that is installed in the Dell, find out which case Acer is using and call up the vendor to verify HDD space allowance (can probably call up Acer to verify this with them or ask them to refer you to case manufacturer).
Alternatively, check what is the typical HDD that is used on that acer model, if the dimensions are the same with your dell, I think they should be able to fit if the connectors are in the same spots.
What I mean is that if you plug in the Dell hard drive into the Acer laptop, it will not boot. It will if your lucky, Bluescreen but thats as far as you will get. You will need to install Windows from the Acer CD. You don't have to delete everything on the hard drive, but programs and what not won't work.