Well, you didn't ask, but I'll tell you how I would do it.

If I already had windows on a partition and I really wanted to be careful, I'd use a rescue CD like rescatux and repartition the HD with gparted.

Let's say you can get by with half the disk for windows. Resize it. Then I'd make a swap the same size to the amount of RAM you have and the rest an Ext3/4 partition for ubuntu.

Next, don't install ubuntu, but boot windows. (The MBR is still the same at this point.) It will tell you the disk is bad, and then fix itself. Now windows is happy and you can install ubuntu on the partition you made.

Of course, you don't need to do it this way. You can use the install procedure to do all that, too. That's what it's meant for. My way just gives you the chance to boot windows first on the new smaller partition and make sure all is well or fix it if it isn't.

There are other things you may consider.

1. Is half the disk enough for what you have in windows right now? If not, make sure you leave windows enough space.

2. maybe even optimize the disk first in windows. I've heard it's a good idea. Never did it myself.

Of course, this is scary, especially the first time. But back up and go for it. Live a little.

Just don't blame me if you bork windows. That's why you back up first.