These companies are often very unhelpful when you have done something different to the computer's native setup.
That's usually because the scripts they work from would not then cover the software problem that you/we may be experiencing.
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I think I may not have expressed my earlier comment correctly.
What I was trying to say was that if the PC needed to be sent back for a "Hardware or damage repair", then it would be irrelevant what software was installed.
Although yes, I would agree that the original supplier need not be helpful if a software problem is being caused by software they have not supplied.
But they would still have some form of commitment, otherwise you would not be able to install any additional software other than that supplied/installed when purchased without voiding the warranty.
As for the Dual-boot recommendation, then yes I would recommend that 100% (hence my own dual-boot systems)