If you have OpenOffice.org installed and opening an MS Office file doesn't call up OpenOffice.org, then you could try the following.
First, copy and paste the following text (between and not including the dashed lines) into a file with a .reg extension (lets call it freedom.reg).
----------------------------------
REGEDIT4
[HKEY_CLASSES_ROOT\.doc]
@="soffice6.doc"
[HKEY_CLASSES_ROOT\.DOT]
@="soffice6.dot"
[HKEY_CLASSES_ROOT\.POT]
@="soffice6.pot"
[HKEY_CLASSES_ROOT\.pps]
@="soffice6.pps"
[HKEY_CLASSES_ROOT\.PPT]
@="soffice6.ppt"
[HKEY_CLASSES_ROOT\.rtf]
@="soffice6.rtf"
[HKEY_CLASSES_ROOT\.XLS]
@="soffice6.xls"
[HKEY_CLASSES_ROOT\.XLT]
@="soffice6.xlt"
------------------------------------
Then, on one machine, do a backup of the registry (just to be safe), then double-click freedom.reg, which will merge the file into the registry (it only takes a second). Then test to see that when you open an MS Office file, it opens in OpenOffice.org. If you are happy, then you can do the same thing on all of your machines.