From what I've been through, finding the "perfect" program for one particular purpose has proven to be quite tough - and it took me about three months to finally come across one that does exactly what I've been attempting to do - merge video files and possibly up-convert them to a higher quality. Yep, you guessed it.
VirtualDub does exactly that, and on the first day that I started using it, I was completely blown away at what it's capable of doing. Merging files/upconversion are just two of many things that it can do, and especially considering that this is absolutely free, this program alone happens to be one of the best that I have ever had the chance of coming across. I mean, as compared to other programs that share similar purposes, VirtualDub puts them into deep, deep shame and pretty much embarasses it since you're comparing a stellar free product to one that's overpriced at $30, and it hardly does anything at all. Gives you poor results, gives you unwanted messages (such as errors), and above all, gives you frustration since you've just found out that you have wasted thirty dollars on a piece of absolute junk.
VirtualDub, from the point of reading its features, has exceeded my expectations because of the simple fact of what it does - it really merges those files, and I mean FAST. I thought for a second that "this is TOO good to be true" and proceeded on to think that the output resolution quality would be slightly lower or even a noticeable difference such that you'll be staring at a YouTube video (usually, the quality of uploaded YouTube videos are crappy, but it's for a reason). Was I ever wrong on that belief!
Disregard all those other programs that claim to be "mergers" since you'll be finding yourself sitting at the computer for a really long period of time (whatever the case may be, you will still find that in order to get those files merged, you're required to leave your computer on for hours and hours, depending on the speed and this is coming from a guy who's on a system of only 256MB's), when all you need is a matter of, say, five minutes? Along with that, you can also save the audio from a video file to convert it to any other format as desire - and as I mentioned before, this is only few of the many things it can do!