Querystring automatically changes

I have an application that's been in production and running smoothly
for 6 months. Recently we started upgrading clients to XP, and users
started reporting problems (one of the pages was erroring out).

Looking at the code, we were passing a querystring value that was in
turn split into an array on the '¿' character. On all other versions
of windows this worked but on XP the '¿' changed to '?' on the
receiving page.

Changing to a different delimiter fixed the problem.
Is this 'auto-correct' feature new with XP?

Does anyone know what's going on.

Thanks.
Serge