In article 7e7dc836-e38e-4064-9087-e4fdf9655de4
@hd10g2000vbb.googlegroups.com, says...

I've just installed Ubuntu Linux 11.04, because Windows kept crashing,
and we didn't get a backup disc with our PC.

Some computers expect you to make your own recovery discs rather than
supplying them for you. For instance, a Dell desktop computer that a
friend has and an Acer laptop that a different friend has, both had the
option to do this from the Start All Programs Dell (Acer) menu. Some
computers only give you a one-time opportunity to do this and once done,
I think that program uninstalls itself.

Other computers, like an HP desktop that another friend has, give you
the option to do a factory reset at boot time, eg, Press Del for BIOS,
Press F10 for boot options, Press F12 for recovery.

Are you sure you haven't got any of those options?