com.apple.desktop.plist
Find the above file. I have OS 10.5.8 so mine is in Users/MyUserName/Library/Preferences
Move the file somewhere else for safekeeping. I usually recommend moving it to the Desktop, but based on what's happening, I'd suggest Documents. Just don't forget where you put it.
Restart. This will create a new plist file in the above location, with all default settings.
If the problems go away, and stay gone, you can delete the plist file you moved.
If the problems still exist, try logging in under a different User Account and see if they're gone. You can also put the original plist file back if you want. That's not necessary if you didn't change any preferences it saves.