There is no guide for this and I am a little afraid of doing something wrong. Is there a guide in somewhere that you could advise or could you tell me how to do? Thanks in advance.

PS. I have a Dell xps m1530

What you need to do depends on your existing configuration. If you only have one partition with Windows Vista on it then during installation Gentoo will offer to partition it. You will be able to manually choose how much space you want to leave for Windows. The rest will be given over to the various Linux partitions required (boot, root and home). Gentoo will then set up an entry in the boot loader so that you can choose to boot windows from there. This should all be automatic.

On the other hand, if you already have a spare partition available then during the installation it may by default propose to use this, or you can always change the configuration to do that.

Your best bet in the first instance is to boot off the Gentoo CD or DVD and see what the installation proposes. If what it suggests is not what you want, then you can post back with what it suggests and we can tell you how to change it.