windows xp - how to get into command prompt when F8 does not work

is there any way to get a into Dos, without a xp disk or if F8 is not working.
have repaired a few blue screens of death with xp disk using repair or fixboot, fixmbr or chkdsk. tried to fix a pc over the phone but friend had no xp disk and just got blue screen.
would like to know if there is away around this.
kind regards

tony