change administrator password from limited account/WIN XP

first check what is the date of windows installation.
if u remember try as follows.
boot PC with Win98 bootable floopy or CD (Partition must be FAT32)
go to c:\windows\system32\config and locate SAM file. and check the date. it must show the date at which you change the password.
now check c:\windows\repair and search for SAM file. it must show the date older than the earlier SAM file. if you find pls replace c:\windows\system32\config\SAM with c:\windows\repair\SAM. all the password and users information will reset and if u know old password of administrator you can login with administrator credentials.