Hello pinky_junsan91,
Can you please tell us the exact edition of Microsoft Office that you are trying to convert from a Trial to a perpetually licensed product?
Also, can you describe in detail the source of the Product Key you are trying to use? Is it printed on a Certificate of Authenticity? Or is it printed on a yellow-orange label that reads "Proof Of License?" Or is it just a hand-written or typewritten product key? But, please do not post your product key!
You might find some valuable information in this Office help article:
http://office.microsoft.com/en-us/help/HA101936151033.aspx
For great advice on all topics XP, visit http://www.annoyances.org/exec/forum/winxp