If you have forgotten the Windows Administrator password, you can either reset or even crack it.
Windows XP stored it username and password information in file named SAM at %SystemDrive%:\Windows\system32\config. The SAM file is encrypted using LM hashes, which is vulnerable to rainbow table attack and bruteforce attack.
I am going to do this with already available tools in Backtrack.
This will dump the hashes in /root/pass1 file as shown below