Did you UNINSTALL the trial version first?
If not, head to Add/Remove programs and get rid of it.
Then try to install your purchased version.
WHICH product key are you using? The one for the trial version or the one for the purchased version?
According to Microsoft "If you are converting from a trial version to a full version of Microsoft Office, you must use the product key from the full version of the 2007 Microsoft Office system."http://support.microsoft.com/kb/823570
Read the entire article for more info.
What site did you download Office from?
You will not lose anything at the end of the trial period. Everything is stored as files on your system,
so they will always be there. There is also a section on what happens at the end of the trial period in the article linked to above.