securing windows...

i am the opperator of a small buissness network. its become an issue that I need a way to prevent people from exiting programs in the tray and processes in the task manager that i dont want them to and changing the clock. I'd also like to keep everything on one account the only log on option available is the welcome screen. when i try to change it an error comes up and says a routine DDL initialization failed. no name no location just failed. after asking around i found out that it may because the version of windows is corrupted. any input would be appreciated. any program suggestions would also be nice.

Look into your group/user policy settings, they have several items there for security purposes and more options that you might not have thought about.