I've just been accessing my Windows disk!

You should have a directory at the root level called mnt. This is
usually empty, and called the mount point.

To mount your Windows disk, open a terminal, and type -

sudo mount /dev/hda1 /mnt (assuming the desired partition is hda1)

Then if you change directory to mnt, ie. type cd /mnt you should be
able to see the contents of your Windows disk, move around the Windows
disk, and copy files into Linux.