Open Control Panel by clicking Start, Control Panel. Click the "Printers and Other Hardware" icon, then click "Mouse." In the Mouse Properties window that appears, click the Pointer Options tab. Unckeck "Automatically move pointer to the default button in a dialogue box." Click OK to apply the change.
If you are using 2000, Click Mouse in control Panel then Motion.... Unckeck "Automatically move pointer to the default button in a dialogue box."