Why is Microsoft forcing all Windows XP computers to install Service Pack 2 by April 12 2004?

Microsoft is NOT forcing SP-2 on any computer.
This FUD possibly originates from a tool Microsoft created at the release of SP-2:
Temporarily Disabling Delivery of Windows XP Service Pack 2 Through Windows Update and Automatic Updates
This tool was to allow Automatic Update to remain enabled so other patches could be installed to the exclusion of SP-2.
When SP-2 was released, some felt more time was needed for users to test and verify their computers are ready before installing SP-2.  The tool expires 12 April 2005

It is easy to prevent SP-2 or any other update permanently.
Simply disable Automatic Update.
Contrary to the FUD now circulating, SP-2 will not be forced on any computer.
There are two ways that SP-2 will be installed:
1. The user manually installs SP-2.
2. Automatic Update is enabled allowing SP-2 to be installed.
If Automatic Update is enabled, there is still one more step necessary for the user to perform before SP-2 is installed.
The EULA must be accepted.
Even if all other requirements are met, the Service Pack will not be installed if the EULA is not accepted.
In both cases, it is an action or setting of the user that allows an update or Service Pack to be installed.

Service Pack 2 is an important addition to the security of your computer.
This tool was only intended to give extra time and not as a permanent solution.
Ensuring the computer is ready for SP-2 can go a long way to prevent problems related to SP-2:
Service Pack Installation Checklist