NO-DRAMA

Hi My Mums laptop running on XP is infected with Security center.It wont allow me to run anything.I have followed some removal instuctions in safe mode with networking as far as running rkill from a disk which stopped the pop ups but it still wont allow me to run Malwarebytes or any other anti virus.It just keeps saying"Widows cannot access the specified path or file,You may not have the appropriate permissions to access them". Any help would be greatly appreciated.Im close to chucking it out of the window!Which I dont think my mum would be too chuffed about!;) Thanks Emma