From Gentoo Linux Wiki

[edit] Introduction

This tip is intended for those who want to reboot to a kernel or Operating system that is not set in grub as default.

[edit] Configuration

You need to have in your grub.conf the following line:

default saved

At the same time, after every menu entry, add `savedefault 0'. Now, you can easily reboot once to a non-highlighted by default operating system by issuing a command similar to:

grub-set-default foo && reboot,

where foo is the number of the entry in grub.conf, counting from 0. So, if you have 2 entries only, grub-set-default 1 would activate the 2nd entry. This will reboot into that entry without you having to wait in front of the screen.

See "info grub", section "Invoking grub-set-default::" for more infomation.

[edit] Credits

The "(OT) Reboot to Windows (using grub)" thread on the "gentoo-user" Mailing List.