Do you have more than one hard drive installed? If you have a master file table that got wiped (likely with a power interruption shutdown), it could take XP up to 20 minutes to boot while it tries to read what it thinks is a raw drive. This doesn't apply if your boot drive MFT got wiped, in which case the only recovery is to take the boot drive to another machine, install it as a secondary drive and recover the data.
Up to 20 minutes? That might be a great tip for me in this case. I'm going to try starting up again, and let it sit there for at least 20 minutes, just to see if this is the case.