first thing. enabling file sharring is bad very bad for most cases. anyways. forinstance if you want to share files between your network then yes you will need to have this enabled but if your not sharing file between your network then no dont have it enabled. it is not a firewall between your two computers ,but it opens lines between your computers and lets things actually happened like viruses spreading between two computers. yes your concerns are like everybody elses for networking just make sure you have a good firewall. as well as a program i like is called black ice. its nice but it does cost some money. its not neccssary but it helps. if you just get a virus protector and firewall you should be good. its an intrusion dection device as well as firewall. but 2 firewalls are better then one. also for you to have a network you need some type of router, switch or brouter. i use linksys. but thats just your choice. anyways if you still think your computer is unsafe. (if wireless) you can make your router have limitations. meaning take the two mac address of your computers and add them to the list of exceptions to allow. if the mac address isnt on the list then the router wont let anything go through. Now, as for gaming that depends will both computers be gaming at the same time or will one be mainly for internet or what ever. if both computers are then you will see some but it shouldnt be too bad. and if it is just increase your bandwidth from your isp. but as for one computer gaming then no you wont have to much lag and if it is too much for you then talk to your isp for more bandwidth. but i hope this helps