|There are two main ways to determine this:|
1. Windows will tell you if you get the 'available wireless networks' page up. Normally the message will say "security enabled" or such like.
2. Plug the PC straight into the router using an ethernet cable, open a browser and dial into the router's configuration screen (this is normally done by putting http://192.168.1.1 into the browser window, though you will have to check as it varies for different model numbers). You should get prompted for user/password (again, the default settings for this will be in the router manual). There should be a setting in there for 'wireless' and this will tell you if security is enabled or not, and even the key that you will need to put into your wireless card application.
If you're unsure how to get into the router, let us know the model/make and someone will get you the instructions (probably me!).