According to
http://www.sysinternals.com/Utilities/PsInfo.html
"PsInfo returns as a value the Service Pack number of system (e.g. 0 for no service pack, 1 for SP 1, etc)."
So you could either mark the step to continue on failure or add the service pack(s) to succeed on to the 'Success exit codes' field.
http://www.visualbuild.com/Manual/programtab.htm