Can any one help me with this problem please

Hello,
i have a dell with windows xp pro. i got this computer secondhand some time ago and it as worked fine untill last week when i stupidley did a system restore on it as it started playing up.now it is asking me for addmin password which i dont have and cannot get. it will not let me on to change password and i dont have the disc to reboot. how can i sort this out without paying out again,