Hi
This OS freezing on Tecra M11 unit has been caused by fingerprint software.
This has been solved by update of fingerprint software….
Found this in this Toshiba document:
Intermittend hang up or freeze of your machine (with possible high rotation FAN) under Windows 7 - 32 Bit / 64 Bit
http://aps2.toshiba-tro.de/kb0/TSB0C032L0000R01