Error 5009 - No definition of "objWMP"

I like listen to Microsoft Webcasts and seminars from the following web
pages:

http://www.microsoft.com/seminar/default.mspx
http://www.microsoft.com/seminar/events/webcasts/ondemand.mspx

Unfortunately, when I am tiring to do that I got the the following
message:
Error 5009 - No definition of "objWMP"

I got this error many times. The only solution that I know is just
reinstalling the whole system.

After that everything is OK for some short period of time (maybe week or
two).

After that I am not able to listen the webcasts at all.
Does anybody know what the solution of that problem is?

I also reinstalled Windows Media Player 9.0 and nothing happened.

Regards,

Andrzej