I don't wanna stir you wrong here because I have not seen HOW the OS's were installed, but typically, just goto CONTROLPANEL > SYSTEM > ADVANCED, then go down to the STARTUP AND RECOVERY box at the bottom and click SETTINGS, then in the next box click "EDIT"
Then edit your BOOT.INI to look like the one in the picture below... this is from one of my customers machines since MY PC is configured for multiboot in the boot.ini.
Just make sure only 1 OS is listed, then when you reboot you won't get that black and white screen that asks you to choose OS's.
Now this is ASSUMING that when you reinstalled windows, you just overwrote the old version, and didn't put windows into another directory folder.
Hope that helps.
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