some features may not work propely with older versions of office.
97 onwards the file ext is .***x whereas the older is just .***
if its likely to be frequent, ie working between work and home, when saving in 2007 when you get to choose the folder etc change the drop down box to 2000-2003 compatible, this will save in the older format .doc etc.
its more a problem in powerpoint and Excel (if using more complex features)
the newer versions of powerpoint have a lot more customisations on transitions etc, so these would need a good check over, sometimes powerpoint puts in somthing appropriate, somtimes just decides not to work.