you have a couple of problems
1, windows machines have too many back ground applications running especialy xp which is the cause for so many ppl having computers crahing and never starting up again, thats why i brought a mac,
2, wpa wep ...... clashes slightly with windows
3, the biggy, only add security to the router
not to the AE's otherwish they can fight each other
imagianne the Airport exprees being like another computer, if you add encryption to it it can lock out, now if you add the security to the router, the ae will have to ask the router to access,
now, being that the AE is realy a form for extending your network.
you ask the AE to ask the router to open up for your pc, ie pcA
now pcB will also need to ask the AE if it can see the router and if pcB gives it the correct password it unlocks the gate for that 1 too
like i say the AE is a network expansion system, if you add security to the router it then become an open close gateway.
the apple store do a wonderfull book which will tell you every thing you need to know about internet and all this kind of stuff, it does regulate around apple products and does suguest some of its own but: the general idea is still the same