Of course, you're assuming that this is a W9x based O/S. Nothing is ever simple.
For W2K:
You can use the TweakUI utility and goto the paranoia tab, or edit the registry HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Cdrom and change Autorun 0x1 to 0x0 to disable autorun. If you use TWEAKUI it will only affect the current user, where as the registry entry will set it for all users. To achieve the same as TWEAKUI change HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer and set the value NoDriveTypeAutoRun from 0x95 to 0xff.
For XP-Home:
Click Start, Run and enter REGEDIT Then go to:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\policies\Explorer
Create a new value called: NoDriveTypeAutoRun
Type is: DWORD
Value is: 0x000000b5 (181)
You may need to log off/log on or reboot for the change to take effect. Additionally, you may need Administrator privileges to edit the Registry.
For XP-Pro
1) Click Start, Run and enter GPEDIT.MSC
2) Go to Computer Configuration, Administrative Templates, System.
3) Locate the entry for Turn autoplay off and modify it as you desire.