It would help if you could provide more information, and Ubuntu 6 is quite old, you should think about upgrading to 8.10. Why don't you download the 8.10 live CD - if you can no longer boot Linux it will at least allow you to recover any data you have in your Linux partitions, it will let you test 8.10 on your hardware, and you can install from it (which is probably the best way to repair your old Linux partition, so everything is up to date).