You could start by pointing out that your i5 has hardly ever crashed because of hardware or operating system problems. Your system availability with an i5 is probably going to be better than with an MS platform.
The i5 has just about everything necessary bundled in the OS (except for your application, which you’ve developed over the years), the MS platform will need various software packages, utilities, databases, etc from different suppliers that may or may not be compatible with each other.
Does your intended .NET software work the way that you/your company want it to work or will it need to be tailored for your needs and how much will this cost/how long will it take? Are you going to change the way you work to fit the new software (sometimes this is easier or better)?