Download and install SugarSync Free on each PC (you'll have to register and send your e-mail address in order to complete the installation), then on each PC tell the software what files and folders you want to keep synchronized. SugarSync uploads those files to your free online backup, and synchronizes them between your PCs. For those, like me, who have a Mac in addition to a PC, it will synchronize between a PC and a Mac as well.
Whenever you make a change to a file that you want to keep synchronized--and you're connected to the Internet--SugarSync uploads the changed file to your online backup, and also synchronizes the file on your other computer. If you're not online, or the other computer isn't online, it waits until you're online or the other computer is, and uploads the file and performs the synchronization then. It's a two-step process: First you choose which folders to back up. Then you choose which folders to sync with another computer. So if you prefer, you can back up folders, but decline to have them synchronized among computers.
There are a lot of extras as well, such as the ability to share files and folders with others. SugarSync also keeps old versions of your files, so that you can revert to earlier versions as well.
Who should use SugarSync? Any PC or Mac user--whether at home or the office--who wants free backup space, or needs to keep more than one computer in sync.
Note: The free version of SugarSync will allow you to keep only two computers synchronized, and has a limit of 5GB online storage space. You can purchase for-pay plans with more storage space, which allow you to synchronize among multiple PCs, and which has faster upload speeds. Prices range from $5 per month to $25 per month.