Since your computers are on the same router, they’re also on the same local area network (LAN). You can therefore keep the folder on one computer, and share it with others. Exactly how to set up the sharing depends on the operating systems involved.
This solution won’t cost you anything, but it’s got one serious flaw: It only works when the PC hosting the files is on, and on the LAN.
Attach a hard drive to the network
Network-attached storage (NAS) puts one or more hard drives into a special box that plugs into your router. Put the to-be-synced folders on the NAS, and every computer on the network can potentially access them (most NASes allow you to control who has access to what).
The NAS, of course, has to be on all of the time, but they’re designed for that and many have power-saving features.
But this is not a cheap solution. Expect to pay a minimum of $150 for the box and a single hard drive.
Set up a local file-syncing program
You can put the folder on both PCs, then use special software to keep them in sync over the LAN. And if you pick free version of Allway Sync, it won’t cost you a dime.
Setting up a local area network can be tricky for the uninitiated, so the simplest solution is the one that avoids the LAN altogether and syncs over the Internet. Services like Dropbox, Apple’s iCloud, and Microsoft’s SkyDrive keep folders in sync even if the PCs are on different continents.
So what’s the bad news?
For one, it’s slow. Compared to your LAN, your Internet connection almost certainly qualifies as a bottleneck–especially for uploads.
For another, it might get pricy. Although these services all offer free accounts, they come with space limitations. If you want to sync more than a few gigabytes, you’ll have to pay a monthly fee. In the long run, that will cost more than a NAS.