Dual boot?

Read the following site:

http://www.psychocats.net/ubuntu/dualboot

Hope it helps.
Peter

Hmm, I understand the original poster had 2 disk drives, one with XP and the other one with Ubuntu.

He probably doesn't want to install a new Ubuntu on the XP drive as described in the link given by Peter.

So I believe the solution is to

1. change your BIOS settings such that the Ubuntu drive comes first in the boot order (Or if your BIOS does not allow to order several disks of the same type change the cabling such that the Ubuntu drive is first)

2. In ubuntu look at /boot/grub/menu.lst

The file has quite good comments an pointers to further help. If that's not enough as more detailed questions here.

P.S. Note that you can render your Ubuntu drive unbootable if you mess up the menu.lst file. So be sure to have a LiveCD
handy before you start editing. If things go wrong you can boot the LiveCD and correct or revert menu.lst

I heard there was an easy way like pluging the XP drive as the master, and the Ubuntu as the Slave, and then when you load XP you can switch to the Ubuntu, while the Computer is running windows. Is that Possible?

> when you load XP you can switch to the Ubuntu, while the Computer is running windows

Yes, that's called virtualization. So you need a virtualization product that runs on your XP host and it has an Ubuntu guest.
Vmware is one such product, but the free VMware player is too restricted for your scenario. So you would need to buy a Vmware workstation license. (and be aware that your desired setup is not the easiest one in Vmware, don't expect to master that in a couple of hours)

Xen is a free virtualization software. I'm not familiar with it's functionality, maybe it could be used.