LILO.
You can edit your /etc/lilo.conf file so you can choose which OS to boot. You'll have to have install LILO to your MBR. After making any changes be sure to type
lilo as root at a command line
before rebooting so your changes will take effect.
You'll need to set up your partitions correctly as well so you can get them correct in lilo.conf. Let's say your partitions are like this:
/dev/hda1 = windows
/dev/hda2 = linux swap
/dev/hda3 = linux
I'm only going to assume that you've got 2 linux partitions. This is meant for example only. I'm not in front of my Linux box right now or I'd post my /etc/lilo.conf for you. I don't dual boot, but you could at least compare it to a working one. Also, I know there's many threads here at LQ with lilo.conf files posted by dual booters. Here's a
search for lilo.conf here at LQ. I hope this helps.