This problem is caused by Ubuntu 10.04 when upgrading the grub boot loader installed in Windows partition where the MBR resides. To fix this, you have to restore Windows' boot loader by using your Windows installation DVD:
1. Boot using the Windows installation DVD.
2. Click repair Windows.
3. It will show you where your Windows is installed. If not, you may have to load drivers.
4. Click Next.
5. Choose MSDOS prompt.
6. At the prompt, type in:
bootrec /fixmbr then press enter.
bootrec /fixboot then press enter.
7. Click Finish then Restart.
Your grub will now be erased and Windows boot loader is restored. When you log-in, Windows will detect other partitions.
Now, how about Ubuntu?
You can restore grub on a different partition other than where your Windows is installed. It is recommended to install grub on your Ubuntu partition. You may search on how to restore grub using your live CD.