Firewall

I need a suggestion for a sufficient firewall program. Preferably free, but one that will not cause me a lot of trouble, especially on the internet.
I am gunshy, I had downloaded Webcelarator, and it caused me to be unable to access the internet or my email. My co,puter ended up in the shop, because I had no idea that this was caueing me a problem.
So I need something which is somewhat fool proof, and will not cause me problems.
Any and all suggestions will be considered.
Thanks.