Well, what exactly do you mean, when you write "shut down
Windows XP"?
Shutting down, is about powering off the system, putting it
into S5 power management mode, which to any lay person,
means it is "turned off", not asleep, so it can't be
awakened because it is off, not asleep. I am lax in all the
various power management abilities, but AFAIK, a system has
to be in S3, no higher, to "wake up" from a power
management event like keyboard.
It could be that you just need to check and change WinXP's
response to the shutdown sequence, that it sleeps instead of
shutting down.
You probably also need to tell BIOS that it should wake on the
USB's interrupt as well.
And you may need to check what is connected to your USB ports.
5 Volt stand-by is supposed to supply not more than 0.5 Amps in
total.
You have now jumpered 5 volt stand-by to provide USB power.
That means that you need to assure that power drawn by all USB
devices combined, in stand-by mode, if supported, remains lower
than 0.5 Amps.
The amout of current 5VSB can supply, varies based on the
specific PSU installed, and whether that PSU rating is
trustworthy. Even so, it is seldom that a system should
need more than 0.5amps of 5VSB. In other words, it would be
best not to jumper a lot of USB ports such that a lot of USB
devices were constantly powered when the system were off,
instead of only the port(s) needed/desirable to wake or
power up the system again. This could mean rearranging the
devices, which ports they are plugged into to restrain 5VSB
to only the port-pair that "needs" it. In general, any
parts that don't need to stay powered, shouldn't be.