If your o/s is win98se, xcopy won't do here either.
You will either have to use image software like Ghost or Drive Image to transfer your OS.
The slowing up may be the result of having a tired o/s and all sorts of bits lying about the drive that you thought you had been rid of long ago
Far better in the long term do a fresh install on the new drive.
Only consider saving everything on the old drive if it is critcal that you do.