My laptop cannot connect to dir-300 wir...

It does sound like you may need to upgrade firmware. DLink is still in the process of updating Vista firmware so be sure you get the right file!. Anyway here is the URL you need:

http://drivers.softpedia.com/get/FIRMWARE/Others/D-Link-DSL-300G-Firmware-R200Mb9cv.shtml

You might like to try:

1. Disable Windows Firewall

2. In Internet Options Control Panel go to the connection tab and then the LAN Settings where you can uncheck the "Automatically detect settings" button.

3. If you have any third party software, I would get rid of this and ensure that the"Windows Zero Configuration" in the Services menu is set to "Automatic" and the status is started.

Interestingly, Anti Virus programs sometimes interfere with wireless set up - you could see if it helps by disabling it whilst configuring your wireless. It might also be worth trying to connect without any encryption code & see if that will connect - you can then sort out the encryption.
You might find this site of some use

http://www.vista4beginners.com/How-to-setup-a-wireless-connection