Windows RT only allowed signed applications to run, and of course Microsoft won't allow signing desktop apps except for their own applications. So the only way you can do (legally) is buying a MS dev account and develop Metro (Modern) UI apps. Or if you dare, jailbreak it and install ported desktop applications. There is a list of ported apps for windows RT on xda.
But there are lots of x86 Windows 8 tablet nowadays, and it's not easy to find a windows RT one. Jailbreak and write desktop apps for windows RT may not worth the effort.