Corrupted boot sector. Buy a new HDD, install XP SP2 and set it to Master. Then take your bad HDD and set it to slave. Then you can copy all of your data back to your new HDD. It is your only chance.
I will periodically 'start over' by putting a new (usually larger) hard drive into my system and doing a 'fresh' install of the operating system and applications BUT I will ALSO put the 'old' drive into that system for a few months until I'm sure I've put everything I need from it onto the new drive.
Something else I'm doing now is I have a second drive in my system and use "FolderClone" to keep "My Documents" backed up onto it. Nothing fancy but it is a form a backup that I don't have to think about.
Good luck.
I'm beginning to suspect you are correct.
This will be the third time I've had to take the same action in the past year or two. I'm glad I have this MacBook Pro running Mac OS X v10.4.7 to use while the PC is down.
The WinXP PC is my main computer for business use, and the Mac is for video editing and such.