From: Travis Siegel 

You can't have two primary *active* partitions on the same disk.  You can
have up to 4 primary partitions, but only one of them can be active at any
given time.  This is what the boot managers do for you.  Using fdisk,
simply make the drdos partition the active one, and ensure the windows
partition is not active, then all should boot normally, and you can use f5
or f8 to step through or skip your startup files.



--- Internet Rex 2.29