We can't know. You have not provided any system information or other details. Sounds like a Flash-based desktop extension tool or a customizable screensaver...
I'm also not familiar with 'WALLSCREEN' and Googling it doesn't return exact results, although, it returns results elluding to 3D/customizable screensaver as Mylenium mentions. Perhaps you installed something that also installed this program. If a program is using Flash Player, Flash Player can't remove that file to install the new version, therefore, depending on the installation type and application, it's required that applications using Flash Player be closed for the installation to continue.