The most difficult thing about building is fixing the mobo into the case and figuring out where all those little wires for the lights go. If you can wire a plug you can probably do it. It's pretty obvious if you RTFM and it's always a kick when it boots for the first time.
I wouldn't pull the mobo from the old box - these things hate being moved and grumble and groan. Keep the old box until the new one is up and going and then use it as a router, keep it as a spare Winbox and install a good Linux distro (Suze 9.0 is nice) and start tinkering with a REAL operating system. You can do all three without losing your Windows installation.
Ah the joys of smb.conf........
