What kind of laptop?
And you should either have "recovery disks" or a "recovery partition" (which is generally accessed at the bios screen by pressing a key, the manual should tell you).
If ya don't have the disks, then I go with Avtrician as the first step. If that don't work, then I afraid I would get a way to boot into dos so I could get the important data off before formatting/reinstalling windows.