It depends on the OS. Because Macs believe in the simple "drag and drop" method of install, most programs don't come with an uninstaller. Theres a good chance that programs long since removed may still have their "extensions" installed and running.

Under OS 9 and below, look for a control panal called "Extensions Manager" and unselect anything not vital to running the computer.

Under OS X, I'm not sure of any utilites built in to disable extensions, but the problem you describe is less likely to also happen due to the modern OS features finally built into OS X.