The short answer is yes.
However, do you mean that you have "unused disk space" as in unpartitioned space or as in extra space in your C: drive (or some other Windows partition) that just doesn't have any files on it.
If you have unpartitioned space, then you should simply be able to partition it with no ill effects to your computer. However, if you have space that is in a Windows partition but isn't in use you should:
*back up your important files in Windows (VERY important)
*defrag your harddrive
*resize the Windows partition to make it a bit smaller
*partition the new free space (as in unpartitioned space)
And any of the tools mentioned previously will do this.