Hi,
Real time OS are OS that are capable of handling real time inputs and provide output in real time. PC OSes are normall not real time. There shall be some delay in processing the input and out put.
Unix and windows on PC are not real time. Embedded linux is available for real times OSes.
The RTOS is the OS which provides deterministic execution time for tasks running there . If application is desinged carefully
for RTOS - you should get tasks execution exactly when you expect them to be executed . Regarding Windows and Unixes
they are not fully RTOS but this does not mean that you can not use them as platform for real time application design . There are a lot of examples where these OS's used for the enterprise appication where real time requirements is a must .
Asa far as I now there are different tasks pri on Windows wehre rreal time pri can be assigned as well . Windows 2k server is better prepared for real time tasks than its professional edition (due to different scheduler behavior ).