Windows 2000 has only got to SP4. I can't remember now the exact message after I installed SP4, but I think Norton Speed Disk might be referring to NT SP5 or something like that (don't know why, may be something to do with w2k using the NT platform).
But whatever it is, it is safe to ignore that message, tick the option don't show this message again, and proceed with Speed Disk.
Every time you install a new service pack on Windows, Speed Disk gives a similar message. The Speed Disk update may or may not come, but it is safe to use it on the new service pack.