I've been trying to reproduce your problem and encountered the same with the 2.5.3 version of standalone Jython. Also tried with POI 3.7; still the same deal.
I also tried the sys.path.append suggestion, Arshad made. Something peculiar is happening here (testing with a different library - this time barcode4j):
c:\development\local\lib\jython-sa-2.5.3>java -jar jython-standalone-2.5.3.jar
Jython 2.5.3 (2.5:c56500f08d34+, Aug 13 2012, 14:54:35)
[Java HotSpot(TM) 64-Bit Server VM (Oracle Corporation)] on java1.7.0_10
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.path
['', 'C:\\development\\local\\lib\\jython-sa-2.5.3\\Lib', 'C:\\development\\local\\lib\\jython-sa-2.5.3\\jython-standalone-2.5.3.jar\\Lib', '__classpath__', '__pyclasspath__/']
>>> sys.path.append('C:\development\local\lib\barcode4j-2.0\build\barcode4j.jar')
>>> sys.path
['', 'C:\\development\\local\\lib\\jython-sa-2.5.3\\Lib', 'C:\\development\\local\\lib\\jython-sa-2.5.3\\jython-standalone-2.5.3.jar\\Lib', '__classpath__', '__pyclasspath__/', 'C:\\development\\local\\lib\x08arcode4j-2.0\x08uild\x08arcode4j.jar']
>>>
See how the path gets scrambled because of the '\' delimiters?
(Also tried the python.path suggestion but it gives the same error as you reported.)
Could it be you're running into this reported issue or something similar/related? It does seem to match the scenario (standalone version) and the versions you and I used.