I have had Microsoft Office 2010 Home and Student for a few days (legal bought copy with key, so that won't be the problem) and right away I noticed Word 2010 could not open a .doc file. By the end of the day this problem was fixed via the registry.
Now I am having similar problems with Powerpoint 2010; double clicking a .ppt file doesn't open it in PP2010 and like the problem with Word 2010- you cannot simply change the default in the conventional manner via the Default Programs menu or the Right click> Open With> Choose Default option.
In the end I found that in the registry; going to HKEY_CLASSES_ROOT\.doc and changing the (default) value from doc_auto_file to word.document.8 the option came up for Word 2010 and it by default began opening .doc files.
All I need is the ppt_auto_file replacement for the registry (if it works) in order to fix it and of course anyone else having this problem with Word 2010 and Powerpoint 2010 will at least have solved one or both of the issues!