Disk Boot Failure shows up when there is something wrong with the operating system may be table allocation files get corrupt or might be anything.

There are number of steps that you can perform even tough format will erase all of your data but this might be a gud solution for your computer to work properly.... you might need a XP  bootup CD or bootup disk.

Toget to DOS you need to either use Recovery Console off the Winxp CD or a windows 98 Boot disk.

You could always try a "chkdsk" in the recovery console if you want to try and keep the documents there on the disk. The file allocation table could be corrupt.