I recommend you use *both* routers in a nested configuration: viewtopic.php?p=326#326
Router 1 will be your Westell DSL modem/router. Router 2 will be your new router.
With the new router 2 disconnected from the network, attach a PC to it, log in and change the IP address to 192.168.2.1. Also make sure the internet connection type is DHCP, and make sure the operating mode is set to Gateway. The operating mode setting can be found under Setup; Advanced Routing; Operating Mode in a Linksys router.
This will create a non-conflicting network segment, since router 1 (Westell) is pre-set to 192.168.1.1.
Then connect a cable from the Internet port on the new router 2 to port 1 on router 1 (Westell). At this point you have both routers on a single network, in a nested configuration. Any PCs attached to router 1 (Westell) can access the internet and any other devices attached to that router, but will *not* be able to see or access any devices attached to the new router 2. Any PCs attached to the new router 2 can access the internet as well as any other devices attached to *either* router.
Connect your PC to port 1 on the new router 2. Log in to router 1 (192.168.1.1), turn OFF the wireless access point, leave everything else as-is.
Log into router 2 (192.168.2.1), set up the wireless access point, and enjoy. Connect all of your other PCs and devices to router 2, which will be secure.
If you ever have a guest over, and want to give them internet access, log into router 1 and turn on the wireless (set a different SSID and channel than the wireless on router 2). You could set weak security, or no security at all, as anybody on that router will only be able to access the internet, and will be unable to see or access any of the PCs or devices on router 2.