Win XP (Home) Start Menu

I have a home-built PC with Win XP Home installed. (It’s a 550mhz PIII with 512 MB mem). I recently installed a DVD-ROM drive (My old CD-ROM died), and ever since, when I click the start menu, if it’s idle for more than 4 seconds (i.e.- my cursor’s not moving over it or selecting something) it closes again. I’m not sure if there was some conflict because of hardware (although I dooubt it), or if the software (PowerDVD) might have been the cause. Any suggestions on how to prevent the auto-closing? Thanx.