macbook with os x 10.5.8 can't pick up wireless networks when other computers can

I've had my macbook since 2007. For the past two years or so it has been a bit slow to pick up a wireless network. My boyfriend has a 2009 macbook with the same software and his computer can pick up wireless a lot faster than mine. In the past year we switched our internet to a verizon wireless card and it's pretty slow internet. My computer has been able to connect, but only sometimes. One thing I noticed is that my computer won't pick up the verizon signal, but when my boyfriend turns his computer on, it will connect and THEN my computer will be able to pick up the signal. We moved again 4 months ago to a house with a wireless router and my computer was able to pick up the signal, but it did more slowly than my boyfriends computer. Then one day, my computer just stopped connecting to the internet. I tried network diagnostics, and my computer was not able to find a signal. My boyfriend computer could connect. When I went to my dad's house with faster internet, my computer would not pick up his signal, but it did recognize some other random networks that were locked. Now I'm back to the verizon card and last night I changed my address to a WPA2 personal and made that network my most preferred. My computer was able to connect! Hurray! But then the next day I was back to the same issue. I'm on my boyfriend's computer now and that's the only reason my computer can connect. Help me out please!