i installed Ubuntu 12.4 inside my windows 7 in a separate partition named "E"
Installing "inside windows" is a wubi install where Ubuntu runs as a program inside windows. If you have Ubuntu on a separate partition, that's a different story. Since you are booting with Ubuntu Grub, if you decide to remove Ubuntu or format its partition, you will be unable to boot without restoring the windows bootloader from a windows installation CD/DVD.
To change the default system to boot, edit the /etc/default/grub file using sudo. There should be a line at the top of the file like this:
Count down to the menuentry listing for windows and enter that number in the file above. Count starts at zero, not one.