The Airport Express is capable of working alongside non Apple routers.
My daughter is at home from university during the summer months, and I had little difficulty in getting her Airport Express to join our existing home network - which has a Netgear DG834N at its core.
All I did was to use the Airport Utility, chose the join existing network option, entered the password for my network, and the two paired.
I did not have to turn 'wifi on' on my Mac Mini either.