As Vocalpoint Castius said, you want PC2 to be your "Server" since this is where you do your file management work. The downstairs computer (PC1) is a "Client".
So on PC2, the Server, you need to set it up so it is running (all the time) whenever you want to be able to access your media files from PC1.
Assuming you have the two computers connected via a network;
1) You go into Tools>Options>Startup and at he "Windows Startup" select "Media Server" or "Media Center and Media Server" (either is okay as long as the Media Server is started).
2) On the Server (PC2) go to Tools>Options>Media Network and check the box for "Use Media Network to share this library ..."
Then go through the setup sets and get the "Access Key".
3) On the Client PC (PC1) go to Standard View and under "Playing Now" expand it to see "Add Library" then type in a name (like "Server Library") and enter the Access key.
That's all there is to it.
EDIT: Just to add - the way I described the setup you are maintaining one library on the Server (you set up Auto Import on the server to monitor the folders where your new media files go, then they are automatically added to the Library). This means your second PC (Client) does not have its own Library and always connects to the Server.
If you wanted to have each PC connect to the NAS and have their own Libraries, you could still set them up as individual "Servers" operating independently with the same settings and views, but then you would have to do file maintenance on both machines - it's double the work and not worth the extra effort IMO, plus you lose some features (like being able to start watching a movie on one HTPC then stopping it, and resuming watching where you left off on the other PC, and sharing a TV tuner, etc.).
