Hi, I'v got this same problem a few days ago. After a couple of days' struggling : re-install the OS, scan the whole harddisc using anti-virus software, etc...Finally I'v made it solved. The cause of this problem is still not clearly understood by me, although most likely due to Trojan spyware, since one of the software I used reported 2 Trojan infections. The pain is that even if after I have removed the infected files, the taskbar simply cannot be recovered, re-installing doesn't work.
Ok, to make it short, here is the way I solved my problem:
1. open regedit, go to [HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer], find the key Streams, delete it. CLose the regestry.
2. open task manager by ALT+CTRL+DEL, under processes, find "Explorer.exe" and end it. [This step is essential]
3. in task manager, File-> new task, type Explorer, a new explorer pop up with new taskbar, and your opened windows are now visible!
You may need to reconfigure a bit about your taskbar, e.g to make the quick launch visible, etc....
IF IT DOESN'T WORK FOR YOU, DO THE FOLLOWING:
in step one, instead of deleting the key "Streams", delete "Explorer" entirely, or simply rename it to , eg, Explorer.old.
step 2 and 3 are the same. But this case you may loose more configurations for your explorer and desktop settings as they would go back to factory default.
Anyway , for the purpose of showing opened programs in the taskbar, It should work.