Boot Windows and go to the Disk Management: click Start, type Disk Manage..., and select 'Create and format hard disk partitions'. There you'll see the 28GB Healthy Primary partition (with no filesystem shown - so it won't show NTFS).
To see what this looks like, check the highlighted partitions in the following picture:
Just right click on the partition and select "Delete Volume". There'll probably also be a smaller one created for Swap. Then you might want to create a new NTFS partition in that space, or merge it back to an existing partition. Since the Ubuntu installer likely created an extended partition you'll have to delete that as well if you want to merge.
Before deleting: make double sure that you are not booting with the grub bootloader or your computer will not boot. It doesn't seem likely given your description, but you should check. Running the bootinfoscript is one way to tell. Another way is to check which menu you see when you boot.
If you're booting with Grub the first menu you see will look like this:
If you're booting with the windows boot loader (and you have Wubi installed), the first menu you see will be the windows boot manager:
To reinstall the Windows boot loader (Windows 7/Vista):
- Boot Windows and create a Windows repair CD
Boot from the CD to a repair command prompt and run:
Make sure your computer boots straight into Windows (or the Windows boot manager)
However, since you wanted a normal dual boot, why don't you download and run the bootinfoscript and post the result to http://pastebin.ubuntu.com/ and then post the link back on your other question. That might be easily fixable. It might even already show up at the bottom of the Wubi grub menu.