The Previous Versions Client allows pre Server 2003 and XP clients to utilize the Volume Shadow Copy Service in Server 2003/2008. Using VSS on a network file share allows your users to recover lost or changed content without calling tech support or pulling out backup tapes. I do recommend utilizing an inexpensive USB HD on you server as the store for your VSS. This allows you the full capacity of your primary storage to be utilized and allows for shadow copies to go farther back in time before being overwritten.
More information: http://technet.microsoft.com/en-us/library/cc757854.aspx
Shadow Copy Client Download: http://www.microsoft.com/downloads/details.aspx?familyid=E382358F-33C3-4DE7-ACD8-A33AC92D295E&displaylang=en