I had a white rectangle on the screen, it popped to the front whenever I switched to Spark, it was displayed in front of Spark so I had to move Spark a bit around. I had time to make a screenshot of it: http://img129.imageshack.us/img129/3998/clipboard11gu.png
After opening or closing a chat window or doing a file transfer it disappeared.
I know that this is not very detailed and I could ignore this as this was the first time it did happen but maybe someone has an idea.
(Spark 1.1.3 + WinXP SP2 + ATI Mobilility Radeon 7500)