First before doing anythinge else, read this:
http://www.makehumancommunity.org/wiki/ ... d_later%3FUsing the old MHX
will cause problems. It is surprising when/if it works, not that it has problems.
This said: this looks like a unicode problem. Did you save the file with a non-ascii character in the name? For modern operating systems it should work, but that XP has unicode problems is not surprising. This would include if any linked asset (texture etc) has non-ascii characters in their names.
If this is the case you could try renaming the files so that they only have the characters a - z in the name.