It's important that you haven't mentioned FATs and FILE FORMATs. Win98 very likely uses an older file-format like Fat16, whereas Win 2000 might allow that, but might not make Fat16 or FAT32 the primary default setting when creating a hard disk partition. It may by default use the non-backwardly-compatible, more-mysterious, less-accessible, more-stable type of FAT, which I think is called NT format, but I may be misremembering. The practical upshot of this is, when you want to have data accessible to BOTH versions of Windows (or to Linux, etc.), you should chose the least common denominator for your hard disk partitions. Otherwise, you may find that Win98 is unable to retrieve info from hard disk partitions with the newer file formats. ---- On another note, you should worry about the dual-boot option. Later on, if you reinstall Win2000 or (god forbid) WinXP, it may refuse to install more than "2" versions of Windows on any given system. I've found from personal experience that reinstalls of Win2k may refuse to install if there are already Win95 and Win2k dual-boot options on the system. --- Finally, be aware that there will be drive-letter reassignment if you add a hard disk or CD or DVD drive to your system. So, for example, if you have an original Win98 install with C: and D: partitions on the hard disk, and CD drive E: ... and then add a new hard disk, intending to put WIn2k on the new disk, then the new hard disk's main partition will bump the original D: partition down the alphabet, and grab the "D:" for its own use under Win98. So, all the old applications may suddenly be able to find their directories, when the hard-drive letters get thrown out of whack. This may result in the necessity of reinstalling alot of programs, or simply allowing shortcut icons to re-locate their program directories.

No, absolutely Not. It is not Mandatory that you should use two HDDS to install 2 operating systems. you can install 2 OS in one hard disk it self. You should specify the Drive to install the specified OS. Dont made your self confused to install two Operating systems in Two Hard disks. Install it in One Hard disk. it will be easy for you. Try this you feel better.

One hard drive. Two operating systems. If you install Win 98 first (Fat 16/Fat 32) You can still install Win 2000 NTFS over it without complications. If you boot to Win 98 then that is all you will see. If you boot to Win 2000, you should see both operating systems.