Now my response to the OP.
When you initially installed CentOS, it should have identified the installed WIndows and added an entry in the grub.conf file. Didn't you see during the install the option about which OS to make the default?
Anyway, now that you have redone the MBR from Windows, you would want to set it up from CentOS. Boot the machine with the install media into the rescue mode. Mount the existing CentOS by following the on-screen instructions. Then you can try the command (for example):
You need to adjust the /dev/sda part depending on your hardware.