First, you need to make files in /dev/sda1 accessible under the running operating system. The underlying operation is called mounting. I think under default Ubuntu settings there will be an entry for /dev/sda1 in the “Places” menu; I don't know how that translates to the Xubuntu interface.
To mount your other installation manually, you can use the following command:
sudo mkdir /media/sda1
sudo mount /dev/sda1 /media/sda1
The files from /dev/sda1 will now be available under /media/sda1. When you've finished browsing:
sudo umount /media/sda1
sudo rmdir /media/sda1
You don't need the rmdir part unless you want to completely clean up after yourself. And the umount will happen automatically when you shut down your computer.
If you'd like to have /dev/sda1 permanently accessible, add an entry corresponding to it in /etc/fstab, which contains information about filesystems to mount at boot time.
sudoedit /etc/fstab
Add the following line near the bottom of the file:
/dev/sda1 /media/sda1 auto auto
You may be interested in the chroot command. It lets you run a program pretending to restrict the filesystem to a subtree. For example, if you run
sudo chroot /media/sda1 bash
you will get a shell where it appears that only the files from /dev/sda1 exist. This shell will run as root; you can use su to switch to a user that exists inside the other installation. Some commands may not run in this environment, complaining about a missing proc; you can satisfy them by running (as root, inside the chroot)
mount -t proc proc /proc
Running GUI applications inside the chroot is possible but the setup is a little complicated.
