We need a few more details:
What is the make/model of the computer? If this is not know, or this is a home-built, what is the make/model of your mainboard/CPU?
You are obviously running Windows XP...Home or Pro?
What were the circumstances that surrounded your computer when it refused to boot the first time? Did you just come home and discover that the machine was no longer bootable, or were there other symptoms that were observed before this happened?
What antivirus software were you using? Halfcalf reported yesterday that AVG had a patch going around that was killing people's boot volumes. If you were running AVG, you could have very well been caught up in this.
Get back to us soon!