I agree with the SyncBackSE recommendation. It's an easy, flexible, and very well supported backup solution.
However, if you want to go with the free SyncBack version, you can get it to back up open files (on WinXP or later) using VSS if you're willing to do a bit of work.
Take a look at the support forum thread: http://2brightsparks...viewtopic.php?t=2801
There's information on using some tools made available by an MS blogger for scripting VSS volume shadows. You can use those scripts to get a shadow started before a backup and stopped after the backup. Voila - backup open files. These scripts should be able to work with most any backup solution as long as you can get the backup to run scripts (or have the backup started by a script), not just SyncBack freeware. They're by no means straight-forward or easy to set up, but they're free.