As your problem seems to be a little bit vague, I try to guess a little bit.
I guess you are in Windows8 and put in the Ubuntu-CD and use the installer. My next guess is, that the Windows Partition is the only one on your Hadrdrive, hence Ubuntu has nowhere to install to.
I takes more to get a Dualboot, then what you seem to be trying (as I said missing infos make me guess, sorry).
If that's the case the following steps are neccessary:
SAVE ALL YOUR DATA FROM THE HARDDRIVE, because if you only have one partition you need to swipe or at least alter the harddrive, which could result in data loss. Take this serious, please.
You need two partitions. You could easily boot up from The Ubuntu CD, install GParted with the software installer, swipe the drive and configure 2 partitions, reinstall Windows and then Ubuntu on the other. Or you use your windows to shrink the existing partition you already have to make room for a second new one to install Ubuntu too.