Best method is 2 format the drive & reinstall the OS
but if have imp data on the OS drive then take a backup
oh!! but u cant logon

then use either:
1) Dual booting method is best (if u have dat) 2 take backups
2) else use BartPe Bootable CD (its pretty good

)
Well after u have done abv things then u can use the microsoft zip recovery method (which i dont remeber so...

)
In this method u have 2 use a floppy 2 take a password recovery which u can refer frm microsoft site....or by google

& u wont be in a problem
then