Synctoy is an excellent Windows XP Powertoy that has loads of interesting synchronization modes.
It was just updated to version 1.2, though I haven't had any problems with 1.1. If you're using it, possibly worth the download.
If you're not using it, and you tend to manually push files from a folder on one PC to a folder on another because you can't quite be bothered writing a batch file to do it and dealing with the synchronization options, you owe it to yourself* to give it a try.
And if you like it, you'll owe me**.
* this statement confers no actual warranty or value, expressed or implied. Value judgements on your person and character are beyond the scope of this blog.
** this statement is binding.