You need to install Windows prior to installing Ubuntu. Windows uses MBR (Master Boot Record), and Linux uses Grub as the OS loader. Linux is flexible, and will install with Windows alongside it, but if you try to install Windows after having installed Ubuntu, you're out of luck. Well, not entirely, but there's a lot of mucking around to do.
Just make sure you have enough space. I wouldn't recommend installing Windows 7 on any partition less than 100GB, from my experience. Because, you want to allow for updates, and whatever software you use, such as games and other stuff. I've installed Win7 on 80GB partitions, and after installing a couple of games, I started running out of space. You don't want to get too close to your partition's end, because that's when you start to experience lag on your OS. Although Win7 does a better job than FAT32 at keeping your filesystem neat and defragged, you'll still run into trouble when trying to find swap space. So do yourself a favor ... Take any backups that you can, and start over. If you can afford to, get another hard drive.
Once you've backed up, boot from your Windows DVD. Format the hard drive, and create one partition for Windows, and the rest of the drive, leave it blank. This way, when you decide to install Ubuntu, it too can create it's own partition. I'd recommend at least 80GB for Ubuntu ... but take that figure with a grain of salt. I use a bunch of stuff, so I need the space. Realistically, you can run Ubuntu with 8GB and be OK, but that's keeping all your other software elsewhere. Perhaps a NAS or Samba share.
If you need further detail on how to do this, follow this link.
Best of luck! Just remember, the figures I gave above are to be taken with caution. If you know you don't need that much, then go ahead and by all means make them smaller. If you need to keep those larger, feel free to make them larger, as I don't know how you'll use your's. I like to estimate and allow for expansion, but if you know how much space your final set-up will take up, then use your discretion.