This is always the topic of a debate when you're putting together a show. Most clubs get a group of people sequestered away and they count the ballots manually as they come in. If you start collecting ballots at about mid-point of the show and have someone bring them in once in a while, you can get it done in not a lot of time.
The advantages of a computer tally are speed of tallying and that you can generate winner's sheets with names, car type, etc that can be used for announcing. You still usually need one person to man the machine and key in the data, which presents a large time-soak.
Most shows I've been involved with in the past few years have been some combination of the two (manual and PC). I have my own thoughts on some alternate uses of the computer, but I won't continue to bore my audience here. Send a PM if you're interested.