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#11
No prob, these are very common questions asked by those of all experience levels.
If you can reimage Win7 onto a partition after installing XP then you should be able to add it from XP using EasyBCD 2.0 which will edit it's own boot files. If that fails, we can help you start Win7 on its own so you can add XP from it, which sometimes works better.
Since XP is Xpiring soon, we encourage installing it to second partition which you could of course do even if installing it first by prepartitioning.
It's frustrating that you can't just install it to the partition you created, but the XP installer is growing creaky and error messages like you got today often don't google to any solutions later than tens years ago. So the error you are getting might relate to pre-SP2 or 3, and solve itself by using a SP3 installer for your licensed version.
I would personally want to try the install first with a fresh XP installer with SP3 for your licensed version, which are available on the web. Just be sure to read the comments to assure it is not cracked or otherwise tampered, and rightclick scan with your AV and Malwarebytes