If you mean, "Is an Excel/Word/PowerPoint file saved in one version readable and editable by the other version," then yes, they are absolutely compatible - that is the whole point. I send files back and forth between work (PC, Office 2007) and home (Mac, Office 2008) all the time.
This.I have seen many people complaining that MS office is even not compatible even within itself
What specific versions and platforms and programs/applications are you looking at?
I have just started using MS Office 2011 for Mac . My word files become a mess in win ms office 2007(my Boss' PC) . Same with PP presentations
Have you tried saving your Word docs as "Word 97-2004 Document (.doc)"
Yeah I would try to change the file type to .doc. Those will open in any version of office I have tried it on and even opens in free word processors like Open Office and Google Docs.