reinstalling micro office 07

I had to reset my computer back to factory settings and chose for my files
etc to be saved. However, it wiped out everything. I have Vista. Which when I
purchased my computer it came with vista and mirco office 07. I ended up with
a "trial" version of 07 when my computer came with the whole thing. Now its
saying I have to repurchase micro office in order to keep it. What did I do
wrong when I put my comp to factory settings? How can I get back the office
07 that came with my computer? I'm a college student online so this is very
urgent that I resolve this soon. Thanks!