If you have more than one p2p sharing program on your computer then they will compete with each for being the default magnet handler.

As for the napster issue. People have sent me lists of files it happened to but I can't seem to replicate the problem. I've been getting "some" fake results but that's all. I suspect if there is something related to napster then as Sleepless pointed out, it's either due to adware or ... there is something within these files that is catching onto a weakness of Explorer.
I've been trying to replicate the problem on my mac, so I'll need to try it out on windows to see what happens there. That could take a while since my windows works very slowly.