You can check status of OS support in the Supported Platforms document in the documenation tab on the support site.
Version 7.4 supports only Windows 8 at this time.
Version 8 has support for Windows 8.1.
WIndows 8.1 is supported starting with 7.4R7 (http://www.juniper.net/techpubs/software/ive/releasenotes/j-sa-sslvpn-7.4r7-releasenotes.pdf)
In regards to Surface RT support, we only support core access and Windows In-Box (not the standalone Pulse client). The error message is in relation to installation of Juniper setup client. What are we attempting to do?