When you install Windows NT-based operating system, the installer will ask you how much disk space to use on your drive. (This question is the Windows fdisk and disklabel tool, all in one.) Tell it 7GB or less, and Windows will create an appropriate MBR partition for itself.
If you wish to access your Windows files when running OpenBSD, format this Windows file system as FAT32. OpenBSD cannot read NTFS partitions. As you find yourself growing more comfortable with OpenBSD, you will probably find yourself booting into Windows less and less frequently, and being able to access that disk space is nice. I know people who started off with dual-boot systems, but finally converted their Windows partitions into MP3 storage without having to reinstall.
Do not attempt to lay out your OpenBSD partition, or subsequent Windows partitions, with the Windows installer! You will quite possibly confuse OpenBSD, Windows, or both. Similarly, do not attempt to create Windows NT partitions (even FAT32 ones) with the OpenBSD installer. Once you have both Windows and OpenBSD installed, you can go in and create additional Windows logical drives.
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