The original OS on C: is Win NT. Now I just install Win 2000 into D:. While I starting PC, it ask me which OS ( win 2000 and win NT ) want to run. The problem is -- even I choose Win NT, but pc still run Win 2000. C: drive still had Win NT, but I don't know why it can not run if select win NT.

Check "boot.ini" in win2k! In the nt351-line, the nt-folder must be there, not the w2k-folder! Expample: [Boot Loader] Timeout=15 Default=multi(0)disk(0)rdisk(0)partition(1)\WINNT4.0 [Operating Systems] multi(0)disk(0)rdisk(0)partition(1)\WINNT4.0="Windows NT Version 4.00" multi(0)disk(0)rdisk(0)partition(1)\WINNT351="Windows NT Version 3.51" multi(0)disk(0)rdisk(0)partition(1)\WINNT3.5="Windows NT Version 3.5" multi(0)disk(0)rdisk(0)partition(1)\WINDAYTO.NA="Daytona 756 (Windows NT 3.5 RC)" multi(0)disk(0)rdisk(0)partition(1)\winnt3.1="Windows NT Version 3.1" C:\="MS-DOS 6.22 / Windows 3.x / Windows 4.1 (98)"

I don't see a mention of what version of NT but if 3.51 it won't, if on ntfs, run anymore. Even with NT4 unless you have service pack 4 or better the older Os won't run after the W2K install due to that W2K converts all ntfs partitions it finds to the w2k version. I suspect when you install W2k you actually overwrote your NT install. In your boot.ini your NT install should be on partition (1) and your W2K install should be on partition (2). If your W2K install is showing as partition (1) then its history for NT.

Last year I had a dual boot, nt351 and Win2k, it was good! You can install both versions on partition (1)!