Looking at the screenshot Spark looks to be a great program. This bug should be fairly easy to fix. I have had this bug occuring before when using a messenger client (also programmed in java) called mercury.
I have the 32bit JRE installed on my computer (latest) although Spark installed its own to the program folder. ALL other programs run fine except the one listed above (e.g Jive Messneger runs fine)
Spark opened and everything was fine. The explorer crashed. I opened Task manager and that crashed as well.
To the standard user this would look like a normal crash. However I have had this happen to me before and it appears to be a java problem. Something in the code or java communicate with something in Windows, causing it to crash.
It may work if I were to install the 64bit JRE and then remove the 32bit one that comes with Spark.
Hopefully someone knows how to fix this so that I can get it working. It isnt worded very well but hopefully you can understand it!