Just tested on XP, it works. Though you need some .NET libraries. If you keep your XP distro updated with the latest .NET and x86 libs, it should work.
Others a problem: it do not open big .vpk (700MB for my test)
Any size VPK will work. There's no progress bar in this version, so it'll take long. 100 MB takes a couple seconds without feedback. Try waiting 30 seconds. Next version i'll just make a progress bar so everyone will know.
Hey. The reason why the window looks a bit bigger than you can reach, is Because It Calculates it's size based on your screen size. It's a square window so it get's messed up When You use it on a screen with an awkward resolution. I'll make 1.2-have a rectangular window so it will look fine on all screens!