Do you mean move EVERYTHING on the hard disk from one PC to the other, including Windows and all the programs and all your personal files?
Well you can (sort of), but read the following.
When Windows is installed on a computer it looks at all the hardware on the computer and installs certain files and makes certain settings based on the computer hardware.
Because each computer has different hardware in it then you cant just move everything on one PC to another PC and expect it to work.
Also Windows makes a note of the hardware on a PC and when Windows is activated it records the hardware details of that PC. If you move Windows to another PC the underlying hardware will be different so Windows will not activate or work properly.
Having said all that, there is software around that allows you to make an "image" of all the files on a hard disk. You can then store this image on an external hard drive.
Later you can restore this image back on to the same PC, but there is nothing stopping you trying to restore it back to a different PC.
It probably wont work properly, and you will get all sorts of errors, and you will need to update all the drivers, but the actual process of moving it from one PC to the other can be done.
Macrium Reflect Free Edition can be used to make an image, and I have used this:
http://www.macrium.com/reflectfree.aspx
Also product called EASEUS can be used to make an image, but I have never used this. I believe there is a free edition.
http://www.easeus.com/
Note to do this you do need some technical knowledge, so if you have never done this sort of thing before you may need help from a friend.