Dont worry the stuff on your hard drive should be safe.
You have a few options. Sounds like you are not happy with the command line so we'll try something else.
If you have your installation CD you can use this as a boot disk, its slower to boot, but it will allow you to boot fully into ubuntu, and your hard disk will be mounted as a separate drive.
Probably the easiest thing to do is to go into this drive and delete some stuff from it that you dont need - make a bit of space.
Another alternative is to move some stuff to a thumb drive to create space on your drive. Remember that when booted from the CD your hard drive will temporarily have a different path. Something like /media/discname/home/yourusername perhaps.
After you have done this, remove the CD and you should then be able to boot as normal.
This assumes that you are correct in thinking that lack of disc space id the reason for your boot problems.