There could be 2 things that are wrong.
1) You have reached the end of the Microsoft Office 2007 Trial.
When this happens you are not able to edit any documents. You will only be
able to view and print your documents.
To fix this you need to buy an office 2007 license from
http://office.microsoft.com
2) You are trying to edit a protected document.
Some people protect their documents or parts of their documents from being
edited. The only way around this is to get an original copy of the file or
the password for the file.
Hope this helps you
($hri...!)