I had this problem too and here's what someone instructed me to do:
this is what he told me to do:
"if you want to try or the others think it is ok the process is:
Open the run command [menu start -> run is normally the bottom command
on the right hand side]
type in: regsvr32 quartz.dll"
That worked for him. At first it didn't work for me, so then what I did was
when I opened up WMM2 I went to options (which is under tools I believe) and
then find the little tab that says compatibility. Uncheck all of those little
boxes and press ok. Then try closing WMM and reopening it, and try to play
your clips in the timeline. I'm not sure if it was a combination of these
steps that worked or what, but I suggest you doing the compatiblity thing
first just to make sure. I hope this helps you!