Dimension 4300-XP PRO SP2

Problems with Mutiple user accounts.With 2 or more accounts only the main account will work [computer admin].Extra accounts-programs will not open,trying to reinstall gives [errors] not authorized to access this program even with {computer Admin} privlege.There are no virus's,spyware ETC.....Memory upgrade from 128 to 384 \ After this upgrade the problem occured.Is this related.Any help is apprec.............