ADMIN. RIGHTS

I AM RUNNING WINDOWS XP HOME ADDITION, I AM LOGGED IN UNDER THE USER ACCOUNTS AS COMPUTER ADMINISTRATOR WITH APSSWORD PROTECTED. BEEN USING THIS SYSTEM FOR TWO MONTHS. EVERYTHING WORKED FINE THE FIRST MONTH, BUT FOR THE LAST THREE WEEKS THE SYSTEM PREVENTS ME FROM LOADING ANY PROGRAM. EITHER OFF OF THE CDROM OR A PROGRAM I HAVE DOWNLOADED. IT WILL BEGIN TO LOAD THEN STOPS. I HAVE TRIED FOUR DIFFERENT PROGRAMS ALL RESPOND THE SAME WAY. I HAVE CHECKED ALL THE SETTINGS ON THE USER ACCOUNT, EVERYTHING SEEMS TO BE SET RIGHT. WHAT ELSE CAN I DO ?