The short answer is you will lose access to Windows if you remove Ubuntu.
When you install Linux, it replaces the Windows MBR (Master Boot Record) with another one, GRUB. The downside of using GRUB is that its configuration file lives in a Linux partition, usually the last Linux distribution installed. So, if you delete this last distro, you lose GRUB's configuration files, and it will stop at a GRUB prompt that is pretty cryptic. It's possible to enter commands here to boot Windows, or any other distro, but it's pretty painful, especially every time you boot.
The usual solution is to use the bootable Windows recovery disk to run a command to replace GRUB with the original Windows boot loader; this must be run from the CD/DVD or repair disk, and can't be run from Windows itself. More info can be found here for Vista and Windows 7 or here for XP.
If you have a copy of the standard fdisk program for DOS/Windows, you can run
fdisk /mbr to fix it.
If you don't have the required Windows boot disks, you may be able to fix it using these instructions.