Make a backup copy of the file /boot/grub/menu.lst
There's info in it that will be useful later.

Then edit the /boot/grub/menu.lst file removing the OS choices you
don't want so the last part of the file looks something like this
(noting Yahoo removes spaces so the formatting is messed up); also
note the "timout" value; 5 seconds (instead of 20) would be good
to boot directly into Ubuntu. When you get that working, then ask
how to blow-away Windows and use its partition for file storage.

[...]
## timeout sec
# Set a timeout, in SEC seconds, before automatically booting the
default entry
# (normally the first entry defined).
timeout 20
[...]
## ## End Default Options ##

title Ubuntu 8.04.1, kernel 2.6.24-21-generic, 21-OCTOBER-2008
root (hd0,0)
kernel /boot/vmlinuz-2.6.24-21-generic root=UUID=4b9e32d4-0db9-\
402c-be33-e9c35be3a81b ro quiet splash
initrd /boot/initrd.img-2.6.24-21-generic
quiet

title Ubuntu 8.04.1, kernel 2.6.24-21-generic (recovery mode)
root (hd0,0)
kernel /boot/vmlinuz-2.6.24-21-generic root=UUID=4b9e32d4-0db9-\
402c-be33-e9c35be3a81b ro single
initrd /boot/initrd.img-2.6.24-21-generic

title Ubuntu 8.04.1, memtest86+
root (hd0,0)
kernel /boot/memtest86+.bin
quiet