I recommand that before you attempt any of this that you make a boot disk for your system to get back into linux. This is just incase anything goes wrong writting the MBR to the drive and your bootloader does not work. If you need help with a boot disk let me know.
Ok well all this will have to be done as root sosu to root at the shell
Then make a backup of /etc/lilo.conf
cp lilo.conf lilo.conf.old
Go edit the file /etc/lilo.conf
At the top there is a line that says default=Linux Label (Sometime sometimes the kernel version)
Change that for the label of what you want to boot by default. The label will also be the same as what is shown on the boot menu when the computer starts, it will be something like DOS or Win2000. Just look in the entries in the file and find the right one, there should not be to many, only one for every entry you have on your boot menu.
Then save that file and at the shell type
That will tell you what the lilo program is going to do in terms of writing to your MBR, if there are any errors STOP NOW and do not continue. You might have messed something up in your lilo config so just go back and take a look.
Once everything comes back alright at the shell type
And this will write the new boot record to the MBR. If you get errors at this stage do not reboot, restore the original lilo and then do lilo -v -v and try to figure out what you did wrong.
I tell you to restore because if every your systems crashes with a bad MBR you might not be able to boot anymore.
If you need anymore help let me know and I will try my best.