How to add a handle to any computer's tower case:
Go to your local hardware store and obtain the following items:
1 handle (larger is better, as it will make your system more comfortable to carry)
4 machine screws
4 6-32 machine screws, 1/2 inch long
4 6-32 lock nuts (the sort with the little nylon disk inside)
4 washers, about 1/2 inch diameter, or as large as will fit next to each other on the holes in the handle
Place the handle on the center of the top of the case, and mark where the four screw holes should be. Remove all components from inside the case, including the power supply. Centerpunch the holes by placing a nail on each of the four hole locations and tapping *LIGHTLY* with a hammer (or, use an actual centerpunch if you have one). Carefully drill the holes using a power drill or a drill bit of suitable size mounted in a Dremel. Install the handle on the outside of the case, with the washers on the inside, and secure with the screws and nuts.
Clean ALL metal filings from inside and outside the case using a brush and vacuum cleaner. Get all of those little buggers out, or you WILL have fried components later! Once that's done, you can go ahead and reinstall everything.
Enjoy your system's new convenience and portability!
There are also handles available at your local marine supply store which will fold down flush with the top of your case. These will require a large hole to be cut in the top of the case using a nibbling tool or jigsaw.