VLC comes complete with its own audio/video decoding libraries, it doesn't use any system DirectShow codecs and so isn't really useful in your case (except to prove that your recordings contain something useful).
When you say "watch other video files on WMP" what do you mean? Try playing one of your GB-PVR recorded videos with WMP, what happens? You could even use GB-PVR Video Library to select your "other video files" and I bet they'd play just as they do in WMP.
Having a lot of available codecs is not a good thing; by doing that your system is sort of at the mercy of the last codec you installed. If I were you I'd uninstall every codec that didn't come with the system, then focus on making one work. It's not good enough to download & reinstall; you need to restore your system to a usable state.
I ran into problems similar to yours after installing one of those silly "all in one" codec packs. Took me a while to recover from that one, but you can do it. I finally settled on a then-current version of nVidia PureVideo for all my MPEG decoding, works better than any other in my system.
If you are interested, you can try a very useful tool for codec evaluation, called GSpot. It can be a bit imposing but it is the only tool I've found that lets me easily edit codec priorities and whatnot.