This is due to the latest Oracle Java security release where they've altered the API to start command line processes, which causes directory names with a space in it to fail. And for J-Express default installation in Windows being under "Program Files", this causes J-Express start-up to fail.
Phillipe fixed his installation by downgrading java to an earlier release/version. The security patch is done both on Java 6 and 7.
We've compiled an alternative Launch.jar file that can be used instead of the original one, that will use another method in the Java API that will handle spaces in directory names. Simply replace your Launch.jar file in the J-Express installation directory with this file:
http://www.ii.uib.no/~kjellp/jexpress/Launch.jarKjell Petersen
Norwegian Bioinformatics platform
jexpress.bioinfo.no support team