JSI Tip 2051. When I booted the W2K CD-ROM (or Setup Floppies), I wasn't offered a choice to upgrade?

To upgrade Windows NT (or Win98) to Windows 2000, you must run Winnt32.exe from the installation source,while booted to the O/S you wish to upgrade.

If you don't, and continued with the install, after receiving a warning that the partition you chose already contains an O/S, you will likely destroy, or at least destabilize, your previous O/S.