I have an 80 gigabhyte hard drive. I would like to replace this drive with a larger drive. The drive that I replace will be used as a external & portable drive to connect to a usb. Is It possible? will It be difficult??

Replacing a hard drive is relatively simple even if you've never done it before. Often, the hard part is opening the CPU case since some manufacturers are dorks in their design. I haven't seen as much of that as of late.

To make the current drive into a portable, you wll need to buy a USB Hard Drive Bay, there are many, and put it in there.

One warning, make sure you are ready to do the reinstall of all your software, drivers, even Windows on the new drive. If you don't want to do that, you will need to use some sort of cloning software and install both drives together at least once so you can clone all the data back on the new drive.