I have a linkysys BEFW11S4 version 4 I am using with a XP PC and a XP laptop. I am unable to get security working with this router. I enable WEP but when I type the passphrase into the laptop it says "security key not valid" even though it is right from the router administration page.
Wait, correction. IIRC, XP doesn't accept a passphrase, only the actual key!
So go back to your router and if you used a passphrase, hopefully it will reveal the generated key based on that passphrase (some do, some don’t). If not, then if it lets you specify a key, then you'll need to specify a key instead, not a passphrase.
Btw, I assume WEP is all you have available on that router? WEP was cracked ages ago and pretty much worthless. It will only thwart the casual passerby. Anyone worth worrying about can crack it pretty quickly. If you have WPA2 or WPA available, I strongly suggest you use it.