Microsoft has stated they will continue to release signature updates for Microsoft Security Essentials on Windows XP after April 8, 2014, but will remove the download page.
I find it odd that they will continue supporting the program but not offer a way to download it.
However, there are two ways to access it after this date:
The program can be directly downloaded for AMD 64 x64 bit users here
and for Intel 32 x86 bit users here
If these links should fail in the future, you can always fall back on the Internet Archive's Wayback machine:
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