i would suggest experimenting with one thing at a time. I would start out with changing the authentication settings to the wireless router.
On one of the computer that has connection to the router, type 192.168.0.1 in the browser's address bar. The router's config page will come up. Since you're going to be using this page a lot, you might want to bookmark the page for easy access.
On the config page, select "HOME" tab and then select "WIRELESS" on the left column. Find Authentication: WPA-PSK and select that. Provide a "passphrase" and make sure you confirm it. It can be any phrase you want to use. Check apply and you're done setting up the router.
Now you have to setup your computer's D-link Utility to match exactly the same authentication. I'm working on a DWL-650 wireless utility and I'm hoping you have the same. Open the utility page and click on "site survey" on the left column. In the middle of the page, you should see all the available network. Select default network and then click on "configure" on the right column. The config page will come up. In the authentication pull down menu list, select WPA-PSK, then select "authentication config". When the advanced security settings comes up, in the WPA PASSPHRASE field, type the passphrase you used earlier in the router's config. Hit OK and that's it for that page.
Now go back to the utility page and find profile in the middle column. you should see a "default" profile. Select that and click on "connect" on the right column. If you see a circle on the small icon just to the left of default, then you've secured your computer.
Basically, whatever you do to the router's config, you have to do exactly the same to the NIC's utility page. If it fails and you have to restore default on the router, there's a small hole in the back of the router. Press a small pin into it for 10 seconds and it will restore the default setting, that is, no security of any kind. Let me know if this helps. Good Luck!