This Python snippet will get the version numbers for the different versions of Softimage installed on the local system.

#http://docs.python.org/library/_winreg.html
from _winreg import *
t = OpenKey(HKEY_LOCAL_MACHINE, r"SOFTWARE\Softimage\CoExistence", 0, KEY_READ )

try:
    count = 0
    while 1:
		name = EnumKey(t, count)
		sKey = "SOFTWARE\Softimage\CoExistence\%s" % name
		t1 = OpenKey( t, name, 0, KEY_READ )
		value = QueryValueEx(t1, "AppVersion" )
		Application.LogMessage( "%s: %s" % (name.rsplit('|',3)[1], value[0]) )
		count = count + 1
except WindowsError:
    pass

On my machine, the script output looks like this:

# INFO : Softimage 2011 SP1: 9.1.91.0
# INFO : Softimage 2011 SP2: 9.2.102.0
# INFO : Softimage 2011 Subscription Advantage Pack SP1: 9.6.194.0
# INFO : Softimage 2011: 9.0.243.0
# INFO : Softimage 2012.SAP: 10.5.98.0
# INFO : Softimage 2012: 10.0.422.0
# INFO : Softimage 2013 SP1: 11.1.1.0
# INFO : Softimage 2013: 11.0.525.0
# INFO : Softimage_2010_SP1_x64: 8.0.249.0
# INFO : Softimage_2010_x64: 8.0.201.0
# INFO : Softimage_7.5_x64: 7.5.191.0
# INFO : XSI_7.01_x64: 7.01.698.0