I second everything before me, as I worked with all of the proposed solutions.
The PowerLine networking stuff works well. I have deployed the older NetGear 12Mb devices (now discontinues) and the LinkSys 100Mb units at several places, including my home. The LinkSys units can even communicate across phases, although doing so increases susceptability to interference -- which my APC SU1400RM2U has inexplicably begun introducing into the circuit which heads my powerline networking, to my shigrin.
I cannot vouch for other brands or models.
The Windows XP bridge idea would work (I have done this myself) though it requires the XP machine always be running, and the WAP in bridge mode would work too, (yup, I have done this, too.)
Now, budget may be an issue. By the time you purchase the LinkSys powerline units, you could purchase a Deneb with a wireless USB adapter. WAP/routers can be had for under $80 these days, and the XP solution would only cost you the PCI wireless card, which I have seen as cheap as $15.