It's unlikely that you'd make your PC unbootable with EasyBCD unless you started playing with the power functions under the advanced tabs without knowing what you were doing or why you were doing it. (It has happened, followed shortly by "EasyBCD broke my PC !"),
Like any power-user tool (regedit e.g.) it is capable of breaking your PC (if for instance you delete all the BCD entries and then try to boot without putting any back), but unlike regedit which just gives you a non-specific catch-all message warning that you
could break your OS, EasyBCD will actually give you a
specific warning "you have made your PC unbootable, don't try to reboot before you add another entry to the BCD" (or words to that effect). Despite the message (in big scary red letters), people have been known to ignore it ("I didn't think it really
meant it"), followed by "EasyBCD broke my PC !" (see above).
I don't quite know the way the mind works for some users, because another scenario which has happened a few times to my knowledge, is users wanting to remove Linux from their boot, perversely deleting the
Windows entry from the BCD and leaving only the LInux.
If you should suffer from some such brainstorm and end up where you fear, have a bootable Linux distro burned to CD tucked away in your desk. You can use it to copy the backup BCD over the one you just destroyed.
If your OS is self-installed, you will also have an installation DVD which will repair the boot for you
http://neosmart.net/wiki/display/EBCD/Recovering+the+Vista+Bootloader+from+the+DVD.
If you are using W7, and you
don't have an installation DVD, you can make yourself a repair CD (Control Panel > Backup & Restore > Create repair disc).
Keep one of those handy
before you run into trouble.