This is what I did in the end:

=COUNT(SWITCH

(Fields!Operating_System.Value = "Microsoft Windows 7 Workstation" and Fields!Version.Value = "11",0,

Fields!Operating_System.Value = "Microsoft Windows Server 2008 R2" and Fields!Version.Value = "11",0,

Fields!Operating_System.Value = "Microsoft Windows Server 2008 SP2" and Fields!Version.Value = "9",0,

Fields!Operating_System.Value = "Microsoft Windows Server 2012 R2" and Fields!Version.Value = "11",0,

False,1))

Thank for all your responses.