Ok...
Have you tried the Secure Empty Trash? Or held the option button down while Emptying Trash to force it to empty without any warnings?
If not it's time to use Terminal, open up Terminal and give these commands [not the comments]:
cd .Trash [takes you into trash]
ls [lists the content, make sure that the Previous Systems folder is in there]
rm -r Previous\ System/ [try without super user strength first: removes the folder and it's content, if you get a permission denied or similar, try:]
sudo rm -r Previous\ System/ [then give admin password when prompted]
Just remember one error (from me or you) when using Terminal, and you could be in trouble... when you use sudo and rm be VERY sure you not have any extra spaces in there... the safest might be to type sudo rm -r Pr and then hit tab to let Terminal auto-complete for you, to avoid typos...