By default, if you delete the C$, D$, etc.. Administrative shares, they will be recreated when you reboot. To disable this feature, edit:
SET AutoShareServer to 0 (zero)
SET AutoShareWks to 0 (zero)
If the entries are not there, create new DWORD values that are named like above and set the two values to 0 (zero).
The information above can also be found in MSKB article # 288164 -- How to Prevent the Creation of Administrative Shares on Windows NT Server 4.0.