why Can't I install the grub on ubuntu 6.06?

[alex]

It must be that when you tried reinstalling ubuntu, you may have chosen 
the same options each time and that gives you the same installation errors.

ubuntu should install cleanly if you choose the correct options.

If you want to preserve whatever data you already have accumulated, you 
can try this:  Keep your current ubuntu and install a second ubuntu  (/) 
to another partition (hda4) so you'll have two installations of ubuntu. 
You don't need to install a second swap.

Use the second ubuntu for your testing--you can try different 
installation options  without losing  critical data in the first 
installation. If by chance you manage to get a proper grub installtion 
in the second ubuntu, you can copy the data from the first one into the 
second, then you can delete the first ubuntu.

All this may sound kind of wacky but it will give you a chance to 
experiment without losing data.