How to remove Ubuntu and put Windows back on?

To remove Ubuntu, you will need a Windows Recovery CD or Installation CD, or a Ubuntu Live CD.
Note: If you don't have a Windows Installation or Recovery CD available, you can download the Windows 7 ISO file (contact Digital River customer support as said in Microsoft help page). You cannot install Windows without a genuine Purchased Product Key though, as these ISO files are 100% legal and will only install as an Evaluation copy for 30 days without a product key.
To remove GRUB:

1. Grab a Windows recovery media or installation CD and boot from it. You should see this on a recovery media CD.
   
   And you should see this on an installation media CD. Click "Repair your computer" and you should see a screen like the first image.
2. Open the Command Prompt, then type bootrec /fixmbr into the Command Prompt.
   
3. Reboot and boot into Windows. Then follow the steps below to remove the Ubuntu partitions.
   (Images from HowtoGeek)

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If you don't have a Windows recovery CD or you are constrained to download and burn the ISO files mentioned before, you can use Boot-repair. It is a tool that fixes most boot problems(Windows or Ubuntu). I suggest using a Windows CD if possible.
To use boot repair:

1. Boot from a Ubuntu live CD or USB
2. Type these lines in the terminal one line at a time.
   

   sudo add-apt-repository ppa:yannubuntu/boot-repair
   sudo apt-get update
   sudo apt-get install boot-repair
   

3. Search for Boot-Repair in the Dash and launch it.
   
   To fix your computer with Boot-repair, simply click the "Recommended Repair" button. Then follow the steps below to remove the Ubuntu Partitions.

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To delete the Ubuntu Partitions:

4. Go to Start, right click Computer, then select Manage. Then select Disk Management from the sidebar.
   
5. Right-click your Ubuntu partitions and select "Delete". Check before you delete!
6. Then, right-click the partition that is on the Left of the free space. Select "Extend Volume". Go through the Wizard and Finish it.
7. Done!

Another note from Tanner: If you are using an extended partition, you might have to remove the big extended partition to make the space unallocated.