Hmm.  Well, in my case, I installed Windows XP Pro, then installed the
NetBSD boot selector.  I got the message described in this thread, and was very
confused.  Then, I began checking to see what the source code said around
the error message, and decided that my problem was that the NetBSD boot
selector was loading the wrong first sector from the Windows partition, and
that it wasn't passing the magic number test.  So, I dd'd off a copy of the
first sector of the Windows partition, by copying it from the dedicated
Windows partition in the NetBSD disklabel, and by copying it from the raw
disk partition.  When that proved that my math was correct, and the two
sectors were  the same, I looked to see if the magic number was right.  It
was.  Then I ran strings against the sector, and, guess what, there was my
error message, along with no other strings.
That's when I concluded that the first sector of my Windows partition
contained a dummy program to just print the error message shown and loop.
A read of several howto's on how to make Linux and XP co-exist confirmed
this theory.  Since Hishashi hadn't solved his problem, at least as he
reported it, I thought I'd try to provide a fix.
	It's possible that installing Windows XP using different options
yields different data in the first sector of its partition, but I'm not
sure about that.  My wife did the Windows install, and I wasn't on hand
when she did it, so I can't say if there were any options that would have
been obvious to check.  I just asked her to make sure she formatted it as
FAT32, and to not repartition the disk, since NetBSD was already on the
system.

-Brian