Rather than removing the kernels, it is preferable to keep them in case you need them one day, ie if an update makes your computer unbootable, you can then boot from an old kernel. You can 'uncomment' them in /boot/grub/menu.lst so that they don't appear on the start-up list.
find the kernels you don't want listed and place a # in front of the entries pertaining to that kernel version.
sudo gedit /boot/grub/menu.lst
Windows is the best virus detector on the market!
Ubuntu attracts Human Beings - Windows attracts viruses and worms