I had this trouble often: while working on datasheet at design time by importing, exporting and editing, all of a sudden, it froze completely even after closing datatable and reopen it. I had to close QTP and open it to work on datatable again.
Windows freezes in that way, that you can't open any new windows. They apear in the process list but without any window until you close QTP.
We found that error in QTP 9.2 they told us, that we have to migrate to 9.5. We did but the failure is the same. So they said, that we have to use QTP10, but nothing changed and the same with 11.0. Have that ticket open on HP since 27.08.2010.