* Use a windows system recovery disc, http://neosmart.net/blog/2009/windows-7-system-repair-discs/
* From command prompt, type "bootrec /FixMbr"
To have the possibility of using Windows 7 besides Gentoo, you can install Gentoo together with Windows 7. Windows 7 should first be installed. To shrink the Windows 7 to a partition, use a Ubuntu install CD. Boot up Ubuntu install CD, choose complete installation. You should here get the possibility of shrinking Windows 7 to a small partition (My is 25GB). Then continue the whole Ubuntu installation process, using the advanced option of not installing GRUB. After completion, you should continue with the installation of Gentoo. Use fdisk, as described in the handbook, to format /dev/sda2 as wanted.
To then boot Windows 7 from GRUB, place the following configuration in /boot/grub/grub.conf:
title Windows 7