With Windows 7 (and likely 8 too, but I haven't checked), you can look at your connections and click the "Show characters" box for the key to see it. If you don't have that ability, you can insert a straightened out gem clip into the hole in the bottom to reset it to factory settings, then rerun the setup program. If you do, WRITE DOWN the passkey in a place you can find.
I have a linsys wireless WRT54GS and I never got my key to my WEP before I changed it from WPA or some such. I'm here now because I have a wireless hotspot on my phone, and my linksys wireless is inaccessible to me.