As root user, edit the /boot/grub/menu.list file.

Find the default boot entry which is usually the first one. You may see a section like:
Code:
title Debian GNU/Linux
   root (hd0,0)
   kernel /boot/vmlinuz ramdisk_size=100000 init=/etc/init lang=us apm=power-off vga=791 nomce quiet fromhd=/dev/null bootfrom=/dev/hda1/*.iso config=scan home=scan noprompt
   initrd /boot/initrd
You must add the nosmp to the end of that kernel line.
Then save it & reboot.
It should boot with nosmp by default.