Can I backup all computers in my network to my server using free software or windows only?

I have lots of different computers with windows (XP, 2003, vista, 7) and I am trying to backup some folders from each computer in the network to server. I want folders to synchronise every few hours!

Is that possible using only windows?
If I need a program, which one?

Answer

You can create a schedule task and use MS's synctoy via commandline to do this, you can also use robocopy which is built into windows and replaced xcopy.

Update

I would wrap my robocopy commands in a powershell script if you want to customize folders and destinations that depend on several factors.

Update 2

Here is the synctoycmd.exe help:

C:\Program Files\SyncToy 2.1>SyncToyCmd.exe /?
Usage:     SyncToyCmd [-args]
   -R           Run all folder pairs that are marked as Active for Run All.
   -R     Run the named folder pair (only one folder pair can be specified
).
   -?           Display this help.

Examples:
   SyncToyCmd -R MyFolderPair
   SyncToyCmd -R

C:\Program Files\SyncToy 2.1>

For the program you want to specify the full path to synctoycmd, and specify your arguments accordingly. It's best to create a folder pair using the gui first, in fact you have to for this to work.