Simple-minded question on folder copy/sync

Hello: i have a home network (ethernet) with an XP machine and a Win7-64bit machine. Want to backup a directory from one to the other (Win7-->XP). I was able to share a folder between them and copy files. Cool. Now I want to backup an entire directory tree. Simply copy from one directory to the other (which is shared and writable), file by file, descending the directory tree. If the target file is the same date or newer, leave it alone, else replace it with the source. If a file or subdirectory doesn't exist on the target, create it.

It's such simple-minded operation, but searching for ways to do this, i come across a myriad of programs, utilities, etc. Dropbox, SyncToy, Acronis, ... Truthfully, this *should* be something you can do with just a simple command line instruction, yes? Or at least, with a file manager - either Explorer, or any other. I'm new to Windows 7, and haven't yet discovered whether DOS-style commands are supported. I would *love* to simply go to the command line, and type something like "dc \usr target:\usr -r" or something of that ilk. But these disk backup applications seem to make this into a very complicated task.

Has anyone else done this? What do you use?

I've checked out Win7's "backup" facility. It does not do what I'd like. Also played around with drag-and-drop copying with Windows Explorer. Nope. It will copy sub-directories, but always overwrites. No option to automatically skip a file if it's the same date (or newer).