Nuts! the Nice Slide Show (& Lite) extensions are the only ones that I offer that still do not have a Joomla 1.6 & 1.7 version. We are working on a new version that is compatible with the Joomla 1.6 & 1.7 systems. However, these new versions are still prototypes.
As you have discovered, installing an extension that is not compatible with Joomla 1.7 can be problematic. Its been awhile since I have solved for this issue. But, if memory serves, Joomla 1.7 makes it fairly easy.
1. FTP to your website.
2. Go to joomla_root_directory/plugins/system/
3. Find and delete the folder named "niceslideshow"
4. Browse to your Joomla administrator
5. Go to Extensions>>Extension Manager and click the Manage tab
6. Find System - Nice Slide Shows in the list.
7. Tick the check box next to System - Nice Slide Shows
8. Click the Uninstall button at the top of the page.
9. Joomla will not be able to find the plugin files and should remove the plugin listing from the Manager.
If the above doesn't work, don't panic. I have another solution that will work. However, it involves a bit more complexity.
Let me know how it goes.
~ All the best