You are using GRUB with Windows and Ubuntu Systems.

After you format Ubuntu, you should recover Windows Bootloader as Master Boot Record, otherwise, you cannot boot to Windows Partition

Insert and boot with Windows Installation Disc.
Press R and switch to recovery mode.

Wait for the count down if you use default keyboard settings.
Select which Windows Bootloader you should recover, then enter your administrator login password.
Type fixmbr and type y to confirm

After fixing Windows Bootloader type exit to reboot the computer and the Windows Bootloader is fixed.