Hello,
If you did a normal install to harddisk then plop boot manager is installed to the Master Boot Record and some space on the fitst track that is normaly not used.
So you won't erase it, but you won't be able to fully undo the install as the saved original MBR info is on that partition.
After first boot of your new installed XP the boot manager wil be disabled as windows will write a new MBR.
As Raider mentioned , if you have a floppy drive you can use a floppy too.