Hi Bogi,
Try deleting the Dock prefs found in ~/Library/Preferences/ (they are named com.apple.dock.plist). Just for the heck of it, I’d try repairing your permissions and deleting your caches using something like CockTail: https://www.versiontracker.com/dyn/moreinfo/macosx/26516 You can also reset or change your Dock preferences using Cocktail, so maybe it’ll work, where the Apple System Preferences aren’t?
If want to dig deeper you can check the following (the Dock application is in there among other things):
“The Dock is located at /System/Library/CoreServices/Dock. If you control-click on it and choose “Show Package Contents”, then navigate to Contents/Resources/, you will find several interesting files. The “Widgets” Dashboard widget, introduced in Mac OS X 10.4.2, is located here, as well as the Dock’s “poof” animation (stored in poof.png), which can be replaced by an alternative animation provided you have an image with suitable dimensions (these can be found on various websites) and superuser privileges.”
Of course, you can always reinstall or archive and install the OS again. A pain, but it’ll work if nothing else. Oh, one other possiblity off the top of my head. Try removing any third party or non-stock Widgets you’ve added.
Good luck, let us know if you find a fix.
Tony!