Well, to leave the firewall normal is as simple as you can make it, auto-decide is like you wont even know it is there.
A simple thing to do to increase its security is by going to firewall's expert settings>>policies>>default rules, set to "ask" for all connections without rules already established.
Firewalls are "simple security", but as you are noticing, you can go far into tweaking one for maximum safety. Change things as you see fit, but never too many changes at once, and always record what you change so you can undo anything afterward.
Here is something to bookmark/read when or if you need to learn more
http://technet.microsoft.com/en-us/library/cc700820.aspx
