If you have Windows 2000 on one or both of the machines it is easy. Connect them using a network (that's a different subject) and then map the files you want to keep sync'd as a network drive on to the other computer. Then choose to make the files available off-line and they will stay sync'd. This maybe sounds a little confusing but in Win2k look in help under offline files and it explains it pretty well.
If you don't have win2k on either machine, then there is no really easy way, at least that doesn't cost as much as buying Win2k. From what I have seen and tried, but maybe someone else knows better...