There is no such button. The BIOS checks the hard drives to see if they are bootable or not. Maybe yours isn't bootable at the moment. So I'd first try startup repair from your Windows XP install disc.

Assuming the PC does boot into another Windows run disk management from there to have a look at that disk. Does it show a good partition?
If you can't boot into any Windows, try Linux or put the disk in an external enclosure to have a look at in on another PC.