Upgrading MS-Office to 2007 results errors

I am using Microsoft Office for my studies and researches. I am currently using 2003 version but since the documents now in ms-word won't open on ms-office 2007 because they have different extensions, I decided to upgrade my office.
But after installing MS Office 2007 and open a document, an error at the bottom of MS-Word 2007 stating "This modification is not allowed because the selection is locked." I opened a file but it cannot be edited.
And when I try to paste something on my doc there is an error that says,

What does this errors means?I clicked the icon Protect Document under the review tab on MS Word and uncheck the Restricted Access tab, but it doesn't change anything.
My computer runs a windows vista home premium service pack 1 with core 2 duo, 32-bit OS. The installation disk I used for installing MS Office 2007 automatically insert the product key upon installing. So I am not quite sure if this is legit or not.
Is there a possible way to fix this problem?
Please help because I already erase the ms-office 2003 from my pc to be able to continue installing 2007.
