Are there special circumstances where this is only a one-time, on-the-spot need? If so, just run it without the WEP for that short time. Chances of anything bad happening are infinitesimal.
If it's a long-term thing, well you can find incredible deals for wireless routers on dealmac.com or dealsontheweb.com. I got my Buffalo Airstation for $20. It's 11-g with wonderful range, even the capability of hooking up an external antenna if that should become desirable. If your time is worth anything to you, then trying to trouble-shoot Windows issues is a vast loss...
Anyway, I seem to recall that getting a Windows PC to connect to a Mac base station required a special character in front of the WEP password, something like $ or perhaps enclosing the password in quotes ("xyz"). It'll be in Apple's KB...