Step by step, to use the existing MBR Partition #1 for OpenBSD, without changing its size (Untested, but may be helpful as a general guide):
- Back up your C: drive and your registry. Be prepared to reinstall Windows, in case you make a mistake.
- Boot OpenBSD install media.
- At the "Do you want to use *all* of <drive> for OpenBSD?" prompt, reply with the default "no". You will enter an interactive fdisk session.
- Edit partition #1 ("e" command). Change the partition type to "a6". Press the Enter key for all other options, which should leave all starting and ending positions exactly the same.
- Flag partition #1 ("flag" command). This will mark partition #1 as the boot partition.
- Write the MBR ("w" command).
- quit ("q" command).
- The install script will place you in an interactive disklabel session. Set up your bsd partitions as you want them. You need at least an "a" partition, if nothing else.
- After installation completes, reboot, using the hard drive as your boot device. OpenBSD should boot. Complete your initial boot, log on as root.
- To boot Windows, run the fdisk program and flag partition #0, write the MBR, exit fdisk, and reboot. To boot OpenBSD, use the Windows disk manager to flag the OpenBSD partition as "Active".
Many people with Windows NT-based systems use Windows boot loader for multibooting, as described in the FAQ. I don't. I prefer a multibooter called "GAG." It is more flexible and easier to set up. You can configure all of your booting partitions on a diskette before committing your GAG configuration to hard drive, too.
http://gag.sourceforge.net