Insert the disk. I assume it will autorun and something like a setup program will come up. See what it offers to install and cancel then.
If it doesn't autorun, look with My Computer what's on it. There might be a setup.exe that you can run.
Even if it autoruns, you can look at the contents with My Computer after the autorun program closes.
Once you know, please tell us what you found.
It's possible you did a default install with the second CD containing pieces of Office not included in a default install. Or it might be documentation like a resource kit.
I'm installing MS Office Professional 2007 which is supposed to give you all the appropriate prompts as you go. It seems to have installed OK and is registered and updated . . . all from Disc 1.
What is Disc 2 for then ? There was no prompt to insert it and continue.