Although Word has a document protection feature you can use but it only protects the whole document. If you want to protect only a part of a document we have to tweak this feature. So, you have to add section breaks around the area to protect. Pretty easy to do. Click just above where you want to start protection and choose Insert > Break > Continuous - this adds a continuous section break in at this point and, because it's continuous there isn't really any indication it's there.
Move to just beyond the area to protect and repeat to add another continuous section break. If you just want to protect the beginning or end of your document you only need one section break.
So far, so good. Now for the protection bit. Choose Tools > Protect Document and in the Editing Restrictions area, click the checkbox and from the list choose Filling in Forms. Now click Select Sections and check the sections to protect (ie leave the ones you want to be able to edit unchecked).
Then click Yes, Start Enforcing Protection and, if desired, add a password.
Now users are locked out of the protected area of the document and they can't change it.
In Word 2007 it works just the same. Find the section breaks in Page Layout > Breaks > Continuous and the protection tool in Review > Protect Document > Restrict Formatting and Editing.