Just reading the following blogpost from my friend Owen's blog about how easy it is to crack WEP wireless encryption. As Owen says its better to have encryption than non at all, if you are unable to use WPA you can also lock down your wireless router by MAC Address. Now every Network card has a unique address which is called a MAC Address you can tell your router only to accept traffic from the MAC address's of the computers on your network.

Now the above is far from foolproof as it is possible to spoof a MAC address using certain Olicom cards and special software. But the hacker will need to scan a lot more traffic to pickup the MAC address of one of the computers on your network and is more likely to move to a network which does not do MAC address filtering. Another trick is to not broadcast your SSID, all networks have an identifier called an SSID if you stop your network advertising itself it will be harder for hackers to find it.