trevor1,
There is a little bit of confusion in here. Windows has its own bootloader and so has Linux. Each doesn't do a bootloader for the other. While you need to twist the arm of Windows to boot another operating system Linux by default will boot every operating system it can find in your computer and natually WIndow is included automatically.
Regarding your first question many Linux users use the dd command to wipe the MBR. Have a look a
this thread
for windows/dos use the old code
fdisk /mbr from a DOS floppy or maybe just FIXMBR in the winxp recovery console.
for linux, just
dd if=/dev/zero of=/dev/hda bs=512 count=1
This approach will render your hard disk unbootable and useless after the bootloader has been removed.
You are far better just using XP's installation disk, delete the Linux partition if you so desire, to install XP which will automatically overwrite the MBR with its own NT bootloader.
In general you can keep as many Windows and Linux in your computer. All you need to know is where their bootloaders are, how to generate them and which one you want it to be the boot manager in the MBR to control the the rest of the operating systems.
The only reason you need to destroy an installed Window or a Linux is likely because you have given too much space for it. My XP is about 15Gb and all my Linux distros are accommodated in either 5 or 10 Gb partitions. Unlike Windows an installed Linux distros can be moved around from hard disk to hard disk and should rarely have a need for a re-installation.