To expand and merge correct responses from above, I'll try to get everything in 1 post:
Ubuntu first, then Windows is generally the reverse direction. Windows won't recognize the Ubuntu, but Ubuntu would have recognized Windows. In the latter dual boot works with no additional work, in the former you need to do more.
In Ubuntu, use gParted to shrink a partition, you'll want at least 20 gigs for Windows 7, possibly a lot more depending on what you plan on doing with it. Apply the changes and do nothing else, do not create a partition in the unallocated space, do not format it as ntfs, do not pass go, do not collect $200.
Windows 7 install will see the existing partitions AND the unallocated space, select the unallocated space and continue on. Win 7 will create the partition and format it as NTFS automatically before continuing on to installing itself.
Once 7 is installed, your Ubuntu will not boot (ie - you reboot the comp, it goes to 7 automatically). To get Ubuntu back, the easiest way (for me) is to download EasyBCD and then add your Ubuntu installation.