How to boost signal from computer?

The router in my apartment building at school is a little too far away. I can connect, but it drops frequently and has a weak signal. I don't have access to the router, and I cannot configure it in any way. Basically, I need a way to boost the signal that my computer is receiving. Is this possible?

Preferably, I would like to use something similar to a cantenna... something I could build myself for basically free. But I wouldn't be opposed to buying something if I had to. Any ideas?