Your hardware key driver is installed in <drive>:\Winnt\System32\Drivers\Sentinel.sys.
The driver installed with ArcGIS 8.1 is Version PD-5.38 (January 15, 1999).
To install the newest hardware key driver:
Uninstall your existing license manager server and reinstall using the ArcGIS 8.1 setup.
OR
Go to the Rainbow Technologies website: www.rainbow.com
Search for the newest drivers for the Sentinel Pro Hardware key and download them.
Reboot your computer.