Is it permissible to repair a computer with a non-genuine Windows operating system if it is not possible to obtain a genuine version, and is that considered theft?

You have done well by repenting from infringing on the rights of others, because the companies that produce the software have intellectual property rights, and it is not permissible to infringe upon them without their permission. Even if some scholars have permitted the use and copying of such non-original software for personal benefit without trading, there is no difference in the prohibition of using copyrighted software, whether you install it yourself or someone else does it for you.