There is a problem with Windows 7 Readyboost function to use a flash memory to boost system performance. Often it will not let you use a usb pen with the error message "This device cannot be used because the Sysmain service is stopped. An Administrator can restart it". Microsoft offers no solution for it, neither can you find it in Windows forums.
I have found a solution: Open Control Panel/Administrative Tools/Services/Superfetch/Recovery and set recovery option for all failures, including Subsequent failures, to "Restart the service". Set also Restart the service after: 1 minute.
After 2-3 minutes of a fresh start Readyboost will work, just wait for it.