Here's how dangerous it is....
Can just anyone see what I am typing at this moment, and what webpages I visit?
Much more likely without WEP enabled.
There are 3 websites I visit whose addresses are not meant to be common knowledge- If I visit them on someone elses network are the websites known to the owner of the wireless unit?
Probably not unless they're running a program to do just that.
How dangerous to my computer security is installing a wireless router?
Depend's who you ask and what you're doing.
Is it possible that my using a wireless network that someone else set up could cause them problems?
Technically or what?