For easy overclock, move over to 12.60 or 12.90 leaked drivers.
 
12.41 drivers are Microsoft approved and received WHQL status under condition of no overclocking or V Sync disabling through registry.
 
Most tweaking programs slip around this limitation built into Nvidia NvCplDaemon function in the Detonator 12.41 by using NvXTInit commands and wreak havoc with the NV registry keys. They also use NvColorInit and often make you lose DirectDraw and OpenGL access.
 
If you still want to overclock Detonators 12.41 get Powerstrip the safest of overclocking program with an effective removal option:
 
http://www.entechtaiwan.com/ps.htm

It seems to be a problem with Windows 2000 and the drivers, and it doesn't seem to make a difference which version of the drivers you use.
 
Wolf87 is correct that you need to be using the correct version of the drivers to get the overclocking tab at all - but you obviously already have that.
 
A search around on the net provided me with loads of other people with the same problem as both of us, but no solutions other than "use Powerstrip"
 
If you find a solution then I'd love to know!

Gainward Cardexpert GF3 and XP. Where can I get hold of 12.60 or 12.90 ?
 
Thanks

Visit the Guru of 3D for Nvidia drivers:
 
http://www.guru3d.com/guide/detonator-explanation/
 
Hercules 3D Tweaker 2.2 is a nice overclocking tool:
 
ftp://ftp.hercules.com/video/exe/NVIDIABASED/P3DTWK-9X2K-22.exe