OS X :: Airport Shows Connection (Full Bars) - Can't Access Internet

This problem has been going on for a couple of months now. I use my macbook at 2 places: school and home. At school, when I click the school network under airport, it connects and shows the full bars. But when I open safari/firefox, the page won't load and I get the error message when you're not connected. However, sometimes after like an hour of persisting and retrying, it will be able to access the internet for a little bit for like an hour or so maybe, and then it'll stop working. At home, it's the same thing. It will connect to my home network (got wireless at home set up through my desktop PC), and show the full connection on airport, but it won't access the internet for the life of me. The difference is I can never gt it to connect unlike at school where it sometimes works, except that it worked for like one day out of the last 2 months (and I didn't change any settings then so it's not like I can attribute it to that). Also, I am absolutely positive that the networks of my home/school computer and the router settings etc. aren't to blame. Reason is that my friends with macbooks can access the internet both using my home network and the school network (the school one is obvious). What could possibly be going wrong? Any settings on my macbook that I can change / completely reset? Is the network card or something messed up? I'm contemplating just completely formatting my laptop to see if it fixes the issue. I have leopard installed btw.