The reason most cataloging is done online is so that you can grab the particulars (cover art, review, cast and crew, etc.) from somewhere else. For example, I wrote my own cataloging program that grabs the information from Amazon. If your HTPC has Internet access, I would either use one of the major DVD cataloging websites or something like DVD Profiler
If it doesn't, then that's a lot more hassle. I did a Google search for "dvd cataloging software" and got a ton of results, so you might look at that. If you just want to store titles and don't care about it being sort of low-rent, you can always use a spreadsheet.