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What is the way to modify the boot sector of a Hard disk ?

Like suppose I install Windows XP on my computer, it makes the hard disk bootable by modifying its disk space. Now, suppose I decide to install Linux on my computer too and have a dual boot of Windows XP and Linux. (Please note that the Linux I am using is Ubuntu 5.04i). Since Ubuntu requires partitioning of harddisk, I decide to install it on a separate hard disk. During installation, it says to install GRUB BOOT LOADER to my first harddisk or gives option to install to a floppy. I decide to install on my first hard disk. Now, my computer is bootable only if my ubuntu harddisk is connected, and I cannot boot from my main hard disk if I remove my ubuntu harddisk. Now what to do in such a situation? Say suppose, my ubuntu harddisk was old and now it has crashed, how can I make my computer bootable back to Windows XP again? I think installing Windows XP again from its CD will do the job but I am not sure. And what to do if I cannot reinstall Windows XP or if I don't have its CD?

I have Windows 98 CD and by it I can manage to go in DOS. Is there any software or tool or tricks available for this problem of booting? Is there any way to modify or rollback the boot sector of my first harddisk?