Patching the ESL on your machine would generally be considered ok. However, distributing the patched ESL would definitely be a violation of your license agreement with Microsoft. In all likelihood Microsoft would not object to people doing that. However, you never know when the lawyers at Microsoft might decide that ignoring it would be setting a bad precedent for the future.

On a different note, of course I'm biased, but I think Y2KFOX is the way to go for several reasons: you can assign the pivot year of your choice to different applications, not one for all applications. Y2KFOX has a "debug" mode where you can actually see the century being assigned. It is has a hot key override to toggle pivoting on and off. It also has a library where you can set the pivot year as desired (assuming you have source code).

Of course, if you have source code, you should take a look at Christoff's solution.

Neil