You do not have to rent out your whole computer, you could simply rent your disk space and then earn some Alt coin and then sell that

for Bitcoin. I have not tested this, but these guys are doing this, http://filecoin.io/ - You install Filebox -- the Filecoin mining client -- and

allocate some disk space to rent. Filebox then stores others' files to mine Filecoin for you. It might be some shitcoin, I have not tried it, so

use with caution.