find another machine that has the identical lsass file and copy and paste Lsass.exe back into its original location, your original lsass.exe is infected anyway so dont worry about it. while you're at it check all your users/appdata folders for wierd names, sometimes they hide stuff in there that execute everytime you reboot the machine.
if that dont work, reimage the machine. save your data if you can. if you have a copy of winternals that will help you.