It is possible for JavaScript to be disabled on the client browser:
Netscape Navigator 3:
Options -> Network Preferences -> Languages -> Enable JavaScript

Netscape Navigator 4:
Edit -> Preferences -> Advanced -> Enable JavaScript

Microsoft Internet Explorer 3:
Edit -> Options -> Security -> Custom -> Active Scripting -> Disable

Microsoft Internet Explorer 4:
View -> Options -> Security -> Custom -> Active Scripting -> Disable

Microsoft Internet Explorer 5:
Tools -> Options -> Security -> Custom -> Active Scripting -> Disable