Why does Spybot-S&D find so many Spyware installers / how is that Download directories setting used?FAQ #015
The Spyware installer detection is the only one that is based only on filenames, without other criteria. The reason for this are two:
1. files not yet installed don't have any registry references to them, and
2. these detection is intended mainly for dialers - files that often differ in contents (different text, different images, different toll numbers) while keeping the same name (within one dialer 'family').

The Download directories setting should be set only to your download folders, where you can easily identify the files you downloaded. Files in other places will be searched for automatically, there is no need to enter them in the setting. In fact, adding your whole harddisk to this setting will most likely result in false positives.
If files like wbemtest.exe in your Windows/System folder are detected as a spyware installer, that is such a false positive.