Lenovo, Dell, System 76 and others sell computer with Linux preinstalled. Linux as a preinstalled option is usually limited to higher end computers as the target market is developers. Installing Linux on a windows computer either as a replacement or dual boot isn't difficult.

If you want to give Linux a test drive download vmware player or virtualbox  create a virtual machine. On a medium or higher spec computer running linux in vmware of virtualbox is an alternative to dual boot. You can have window and the linux vm open at the same time on the same screen or each running full screen dual monitors.