This can be coursed by several different things, please read the list below.

1)  To Alt-Tab in SSPro, you will need to press the Alt key first, then wait for the OK TO TAB SCREEN, then hit the Tab key.  This is due to the fact that SSPro needs to set its self up for Playing in the background.

2) Microsoft Office task bar TSR program: If you have this loaded, you will not be able to Alt-Tab back to SSPro, only the OK TO TAB SCREEN.

3) There are lots of different TSR program's that generate interrupts or waist system resources, please unload them one by one until you find the course.