If you're using boot.ini you're running WinNT, XP or 2k or later?
To boot from the disk of your choice you need to know the physical channel, disk and partition of the OS you want to boot. You can then Edit the boot.ini directly using edit under Dos or open it in notepad under your existing OS. When you save it it may ask about format changes which - as long as the edit you've made is consistant with the format of your existing boot.ini file you can ignore.
Note the boot.ini lives on your machines C drive or the first active partition the bios finds together with the io/sys, ntldr and all that good stuff. You can't move it. Also note that those files are usually protected and or hidden so you'd have to change the attributes or set your folder options to view all hidden and system files.
The Boot ini format would look something like this;
multi(0)disk(0)rdisk(0)partition(1)\WINNT="Windows FIRST DISK" /fastdetect
multi(0)disk(1)rdisk(0)partition(1)\WINNT="Windows SECOND DISK" /fastdetect
When you boot it will show the prompt with the options of the systems you've named in the quotes. If you don't anser in 30 seconds it will boot the system on disk one partition 1.
Copy yourself a backup before editing. Make sure you have a system ERD diskette...