Having never used Xandros I can't really tell you how it would work on there.
I can, however, tell you with Slackware. During the install process you have
two options that can alter your present drive. The first is to run the cfdisk
program, which is a curses based program for partitioning any hard disk drive.
With cfdisk you can setup your partition tables, with a lot of variables.
The second thing is that during the setup you have a TARGET option which is
where you choose the partitions where you want to install the OS. With each
one you choose there's the option to format the partition. This doesn't totally
erase the disk, as a low-level format would do; or writing the drive to
zeroes. But for normal purposes it will get rid of your old installation.
If you want to totally erase the drive, such as you would do if you rubbed
a magnet against a floppy disk, you should determine the make of your hard
drive. Then browse to the manufacturer's website and get a disk utility so that
you can do a proper low-level format, or write the drive to zeroes. If you're
interested in doing that, and need more help, post back.
There may be another method, such as formatting before installing with some
open source app, but I'm not aware of it.