well i can't actually tell u everything,ill just tell u what i know,cause when i try to open any sistem program it gives me an error.
Its windows xp sp 3
Are you stuck at the logon screen or can you actually log into windows, but it will not let you run programs?
if it's case 1, then you need to get into administrator through safe-mode and reset your password, if you forgot the password to that, then the only way you can reset it is by running a bootable ISO that is capable of retrieving your password, linux live distro has a few, and there are others out there...
if its case 2, then it will depend on if you set it in administrator mode or not, if you set it on purpose to restrict certain programs, again you will have to go into admin^ as above, but if you didn't, then you're gonna have to either a)system restore, b)repair install from a boobable windows disk...