Blah19,
you are far better off replacing the laptop HDD than trying to boot from an external one, particularly since your laptop does not have the option to boot from an External HDD. It would be a whole lot of mucking around, and quite honestly ain't worth it. if your going to boot from an external device use either CD or USB key, CD would be the way to go and then use that in conjunction with a USB key to save your files as Nuxology has suggested.
you might also want to check out this link, however I cannot garentee this will work ( it is 5.10 they are doing this with) :
http://ubuntuforums.org/showthread.php?t=80811
you could also look at UbuntUSB, it allows you to install Ubuntu on an External HDD with no Bios setting whatsoever, it costs though 29EUR which is about $37US
http://www.zinside.com/index.php?mai...79&language=en
also have you tried turning the machine on with the External HDD connected, if you go into the bios it might show up as another Drive, then you can boot from it I've Seen it done before. Just a thought
ciao,
Jake