One question I hear pretty often is: "hey, I have two machines with OneNote - how do I keep my notes in sync?" There are a lot of ways to do this. In some ways it depends on what your needs are. After all, what doesn't?
First, be certain that you have Sp1 of OneNote. Use Help/Get Updates to check if you are not sure. SP1 has some adjustments to perform better when using files across multiple machines.
Next, decide if you want to sync all your notes or just have a portion of your notebook available on both machines.
Next decide if you need to have the notes accessible when you do not have access to your network.
Now, here are some typical ways:
Use Windows offline files
With this scheme, you will designate one machine as the "server". The other machine(s) will be the "clients" (there can be more than one client). If you have a laptop and a desktop, either one can be the server. They just have to be able to see each other on your network. When the client gets onto the same network as the server, it will connect and synchronize any files that are different between the two. While it is one the same network, OneNote on the client will work directly against the files on the server. The thing you must avoid with this setup is making changes to the same OneNote section on both the server and the client while the machines are not connected. If you do, there will be a conflict and you will have to manually resolve which file to keep.
On the server machine, make the portion of the notebook you want to share into a shared folder. For example, if you want to sync your entire notebook with a second machine, make the folder "My Notebook" a shared folder (right-click on the folder, choose sharing, etc. Note one gotcha is that you not only have to give yourself read/write access, you also have to give yourself security permissions to access this share. If you don’t know how to do this, consult Windows help.) I should note that of course you can also use an actual server for this, and make both machines clients of that server.
Now, on the "client" machine, connect to that share (e.g. \\mastercomputername\notebook). Now, make that folder available offline. If you don't know how to do this, first read this article: http://support.microsoft.com/kb/307853/. Now, read this article on how to set up OneNote using this method: http://support.microsoft.com/?kbid=831596. Keep in mind that this second article provides the most secure steps to avoid any trouble. Most of the time everything works fine without being religious about closing and manually syncing. Best to follow the steps in the link though if you plan to do this a lot and edit a lot on both machines.
If you have a different sort of setup, such as a desktop at work and at home, plus a laptop you carry between home and office, you can consider some options;
Use a file syncing program
There are lots of utilities that will sync files between two machines. You can use these to make two folders have identical content when the machines reconnect with each other. This is similar to Windows offline folders. These tools have the same limitations in that you cannot modify the same files (sections) in two places without syncing first. Some of these tools let you pick which file types to synchronize. You can opt to not synchronize the hidden *.onetoc files which OneNote creates in each folder and which hold the information on which sections are open or closed in that folder. If you do not sync those, you can have a different set of sections open on each machine even though the set of *.one files is the same.
Use a "real" server
You can place a single section, multiple sections, or an entire folder (with subfolders) onto a UNC server share (i.e. normal file share using the \\server\share syntax) or multiple sections onto an http:// location that supports WebDAV (http://www.webdav.org/other/faq.html) Typical servers that support WebDAV you might be familiar with are MSN Groups and Windows SharePoint Services, as well as any of the "hard drive in the sky" websites.
For example, I keep one section on my personal website (my hosting service supports WebDAV), and all my machines and my wife's machines point to it. I set this up just by saving the file to the web site (Use File/Save As, then paste the URL of your web site into the Save dialog). This adds to your set of sections a "Shortcut section" tab to the section file which is now on the web site. You can go ahead and delete the original now. Then from every other machine I just did a File/Open and navigated to the web site, and opened the file. Of course each machine needs to have read/write access to the location where the section lives.
The difference with this system over the offline files or file sync approach is that the data always lives on the server. If you cannot see the server, you cannot see the data. Conversely, there is no conflict danger since only one person can change the file at a time. This is a good solution for desktops that are always connected to the internet or your internal network.
Create a shared notebook
What I call a shared notebook here is just a folder share that is accessed from many machines (yours or other people too). It works only with UNC shares. First, set up a share like \\servername\sharednotebook. Now, use File/Open Folder, and open that folder share. That's it. Tell everyone you want to work with to do the same. Do it from all your machines. Now you have a shared OneNote "notebook" that shows up as a folder in your own notebook. You can't take it with you since it lives on the server, but you can work on any section, create new sections, etc and every user sees the same stuff on their machines (in addition to their private notes in other folders). While you work on a section you prevent others from editing it, but after a minute or so of inactivity others will be able to work on it. If you want to hurry that up, you can always right click on the section tab and choose "Allow others to edit".
Let me know how it goes!