General Networking/Lan/Wan/MAC Address
Can anything sinister be done to my computer if someone has my mac address?
Not usually no. Your MAC address is only visible on your LAN, not the Internet. On the Internet your PC's IP address is usually hidden also - behind the IP of your router. A MAC is a local and Unique hardware address that is bound to your Ethernet NIC or your Wireless adapter. You can see it if you run "ipconfig /all" at a CMD prompt (start, run, cmd <enter>). The first half of the MAC is the Vendor that makes it. The 2nd half of the MAC is the unique address of your device.
Your local router sends packets to your MAC/IP. But Its MAC in turn (on the WAN port of your router), is what it uses to advertise to the next hop router.
Someone on your LAN can see your MAC, but not the Internet. Think of a MAC as kind of like your house address - it is fixed and doesn't change - so that mail can get to your house. But your phone # is like your IP address, can change, and it is tied-to your MAC, for network traffic.
Anyway, there's a lot to the whole IP and MAC thing. But you shouldn't worry about your MAC - it can't even be seen from the Internet. I don't even bother to use a firewall, anti-virus etc on my PCs - just the firewall on my router. As long as you are careful about what emails you open, it is pretty rare these days to even worry about security at home, unless you've got someone pretty slick inside your house - on your network.
I hope this helps!