1. First of all you need to connect the Desktop to one of the LAN ports on the Wireless router.
2. Second, you need to connect the WAN port on the Wireless router to the Network port on the Cable broadband router.
3. Now, on step 4 on the Wireless router page, choose Static as opposed to DHCP and plonk in the static IP settings assigned to you by your ISP including the DNS addresses.
4. On the Step 5 on the Wireless router page, I suggest changing the SSID to something more personal and recognisable such as Gas_home etc.
5. Now looking at the Wireless Router setup page, click on Wireless on the left hand column and find the Security/authentication setting that should allow you to choose WEP etc.
6. If available, choose WPA-PSK TKIP or WPA2-PSK AES encryption and specify an access password (at least 8 characters). WEP is a pretty weak encryption and you will often have connection problems.
Let me know how it goes.