Cannot access Windows Explorer or even Computer

System: Dell Inspiron 660, running Windows 7, hooked to an Asus VG23AH 23in widescreen IPS LCD monitor, which has four modes--HTMI 1, HTMI 2, DVI, and VGI..

Click on the Windows Explorer icon on Task bar, the icon will just disappear to the right of the screen. Sometimes, but not always, I can retrieve the runaway Windows Explorer in VGI mode, and work on it from there. The same thing will also happen to the Picasa icon on Task bar. 

Any help will be appreciated.

gch