Ok, let's start over from the top:
A) What container does the file use? Is it AVI or MKV? If it's AVI, 99% of the time you're just going to have to crop them out. If it's MKV, then you'll only have to do that, say, 15-20% of the time, if that. This is because MKV can easily do softsubs; AVI can do them too, but it's hacky to put them in AVI and practically no-one releases videos like that.
B) When you open the video in VirtualDubMod, what
does show up in the Windows' taskbar (where the clock is)? If it's a little red icon that says FFV in even tinier letters, then ffdshow is handling the decoding. If you double-click on that icon, the Video decoder configuration dialog will come up. Alternately, you can get to said configuration by clicking on Start, going to the Programs menu, finding the ffdshow section, and using the Video decoder configuration link there. If neither one of those things can be found, you don't have ffdshow installed, and can download it here:
http://sourceforge.net/project/showfile ... _id=173941C1) DirectShowSource, as the name implies, uses DirectShow (one of three main Windows-based media frameworks, along with Video For Windows and as of Vista/Win7, Media Foundation). AVISource uses Video For Windows to do its dirty work. I don't know of anything that uses Media Foundation yet. Basically what this all means is that if you can play something in Windows Media Player, you can use DirectShowSource to open it. If you can open it in VirtualDubMod without using a script, you can use AVISource*.
*if the file is actually an AVI, anyway.
C2) FFmpegSource, on the other hand, uses FFmpeg (another media framework) to handle its decoding. The difference is that FFmpeg originated on Linux instead of Windows, but is pretty much cross-platform. There are versions of FFmpeg on Windows and OS X in addition to the many different distributions of Linux you could call its 'home turf'. The difference between FFmpeg and the Windows solutions described in C1 is that all of FFmpeg's decoders are built-in, and the work to support additional ones is closely monitored by the development team. The two main cores of FFmpeg are known as libavcodec (the actual video decoder), and libavformat (the part which works with the containers). libavcodec has been used as the core of several media players such as VLC and mplayer, and was ported to DirectShow as ffdshow. The major benefit here is that if you make a program with libavcodec, it's all self-contained. FFmpegSource, both the old 1.21 version and the new 2.00 beta versions, use libavcodec to do their decoding, and don't require anything else to be installed (so in other words, if VLC can play the video, FFmpegSource can more than likely work with it too - it just doesn't require VLC be installed first). Unlike DirectShowSource and AVISource, though, it doesn't come with AviSynth by default. You have to go and download it from the page mirkosp linked to, un-RAR it, and place the plugin (FFmpegSource.dll or FFMS2.dll) in AviSynth's plugins folder.
A2) I put this down here because it has to do specifically with MKV files, and while this is pretty much cut-and-dried, it's more wasteful of hard drive space than simply using AviSynth to open the file with FFmpegSource.
If the file is an MKV, you can go and get MKVToolNix from
the official website. After installing it, open the MKV file in it. It will show you the various things (video, audio, subtitles) that the MKV holds. Uncheck the box next to the subtitles and audio, or just the subtitles, change the filename or location in the Destination box, and then click Start muxing. It will create a new MKV file that simply doesn't have those streams in it anymore. If you open that new MKV with DirectShowSource in VirtualDubMod like you'd done before, then you won't have any subtitles (or at least, you won't have the subtitles contained in the stream you disabled; there is still a trend to hardsub karaoke effects in OP/ED sequences or random signage inside the episode, and unfortunately you'll just have to crop those out if they're still there).