But, are you saying that the computer has had these problems since the first day, or it only started recently?
If it's from the beginning, it IS definitely their problem, because they would be selling you a faulty computer (even being on the software side).
If it only started recently, and you installed softwares by yourself (not the store or from factory), it might be only one program that is not compatible either with Vista or with some other previously installed software. In this case, maybe it's a good idea to keep a list of programs you install, with dates, and if you start getting trouble, you might have an idea of what is the problem.
Before installing any program, also take a look online on the program's website (if it has one) if the program is compatible with Vista. Not all of them are. For example, my sister had a webcam that worked perfectly with XP, but when using with Vista, she only got a black screen, because the drivers weren't compatible with Vista.
One other thing: you said that the warranty would be broken if you took the laptop somewhere else. Well, if the warranty doesn't cover software, then they cannot cancel it if you take somewhere else to fix the software part, I think (as long as the other place doesn't touch the hardware parts). Maybe someone with more legal knowledge could read the warranty papers and evaluate that?
I hope everything works fine now.
malu