Windows 10 *is* available on any our custom built systems but only as your choice. We have a very limited amount of Windows 7 Pro for the time being as the reliability and support are virtually unmatched with any other version of Windows.
Windows 10 is less problematic on clean installs but still not as trouble free as Windows 7. As for support, Microsoft will be support Windows 7 until at least 2020. The ‘free’ upgrade to Windows 10 is still possible if you have a Windows 7 COA (the sticker on the side, top, or back of your PC or bottom of your laptop. However, if you have Windows 7 you will often be happier sticking with it!