Do you not have the password to your own BIOS? YOu could always reset the BIOS - there's often a switch or jumper near to the battery (have a look in your motherboard manual), or you can simply remove the battery for a short time.
That doesn't help you start the computer however, if no operating system is being found. Do you have a recovery disc? Try booting with this in the CD drive. This is where the BIOS tip might be handy as you will be able to direct the machine to boot from the CD first, rather than the Hard Drive which is causing it problems.
At worst - though you've a few things to try first - a complete reinstall of XP may be required. If you have your current XP Serial Number then you might be able to borrow an XP Disk and enter your number when appropriate (your recovery disk, if you have one, may allow you to do the same). No recovery disk and no serial number? Looks like you might have to purchase a new OS.