Don't worry. The bootloader can be repaired even if you completely destroy it. You just boot up with the install cdrom and fix it manually. Nothing too important is kept in the bootloader part of the MBR (the partition part is important though). Both Windows and Linux only put enough code into there to initiate the boot up sequence and give you a rudemtory interface (the menu). All the important parts are kept on the OS partition itself.
If your using grub your lucky. You can just edit the config file and the changes are automatic, however if your using lilo you have to rerun the lilo command and you have to use the chroot command to change over to the OS on your disk to do that.
Grub also has a rudementary command line interface. If your config is messed up you can access this command line and give it the steps manually.
Here is a basic grub config (also keep in mind that to programmers 0 is the first number, so the counts go 0 1 2 3 4 5 6 7 8 9 instead of 1 2 3 4 5 6 7 8 9 10 like normal sane people do):
default=0
timeout=10
title Red Hat Linux (2.4.7-10)
root (hd0,1)
kernel /boot/vmlinuz-2.4.7-10 ro root=/dev/hda6
initrd /boot/initrd-2.4.7-10.img
title Windows XP
rootnoverify (hd0,0)
chainloader +1
Here is redhat discussing grub in it's wonderfull manual
harddrives are the hd stuff. So (hd0,0) to grub means the first harddrive and the first partition, (hd0,1) is the first harddrive and second partition.
Which is different then what Linux sees. You can think of grub as a mini-OS that has it's own weird stuff.
I won't go into the command line interface, because it's awkward to use if you never seen it before, but it's a real convenience to use if you did a typo. You can just edit the config entries and fix the typo from the command line. You can even add completely new entries if they are missing (the changes won't be saved though until you edit the config)
But to fix the config file you just boot up with the install disk and you need to get to a command line interface somehow. Usually you can switch to a virtual terminal easily by hitting ctrl+alt+F# were F# is one of the F-numbered keys. Probably ctl+alt+F2 is your best bet.
Once you get to the command line mount your harddrive's partition to any directory. Probably mnt, but it can be /mnt/hd or whatever you want.
mount /dev/hda2 /mnt/hd
In linux harddrives (and other IDE devices like cdroms) are represented by special /dev/ files. hda is for primary master, hdb primary slave, hdc is secondary master, and hdd is secondary slave.
Each partition is it's number tacked onto the end of the drive letter. Like /dev/hda1 or /dev/hdb3
Then you just cd into were the grub config is located and edit the file to correct it. Next time save a copy of the original file, just in case you mess the real one up.
Then once the changes are made, umount the mounted partition and reboot. Hopefully it's fixed and that's all you have to do.
The proceedure to fix lilo is a bit different.
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