Wireless Security Options For Mac Os X 10.4.11?

Have a mac os x 10.4.11and we use the Verizon Wireless Broadband Device which plugs into the computer via usb port.I was wondering, my connection works great with everything, uploading pics, internet blah blah blah.But when it comes to downloading large files...it wont. the connection times out cuz it takes hours.Never did this when we had regular internet through cox. It was always pretty fast. Could i be in a bad connection spot, or is someone airport mode on my wireless connection or something like that?How do i make it so no one can use my wireless...password protect it? thankyou for your help!!