You can't delete the local administrator account; Windows won't allow that. What you probably have is the local administrator account is disabled which is the default behavior for security reasons. First, try to use SystemRescueCD (http://www.sysresccd.org/SystemRescueCd_Homepage). This has a utility called NTPASS that allows for the recovery of local user account passwords (you can change or delete a password, not really recover an existing one). See screenshot below. Another method is to do an in-place install taking care to preserve files, but that may be problematic. If all else failes, you will have to use DominicP suggestion. Either mount the disk via an external drive or use a Linux distribution like Knoppix or Ubuntu (live CD) to recover data.

Is there any way to get in to 7 without any account? There's a student who have forgotten his password, and the administrators account is deleted. Please help he has a lot of work on the computer and he dosen't want us to reinstall the os.

You can either remove the hard disk and attach it to a working system or use a live CD\DVD\USB stick, e.g. Knoppix http://knoppix.net/, to access the disk and copy the files to an external drive before reinstalling the system - assuming the hard disk is good.

You can't delete the local administrator account; Windows won't allow that. What you probably have is the local administrator account is disabled which is the default behavior for security reasons. First, try to use SystemRescueCD (http://www.sysresccd.org/SystemRescueCd_Homepage). This has a utility called NTPASS that allows for the recovery of local user account passwords (you can change or delete a password, not really recover an existing one). See screenshot below. Another method is to do an in-place install taking care to preserve files, but that may be problematic. If all else failes, you will have to use DominicP suggestion. Either mount the disk via an external drive or use a Linux distribution like Knoppix or Ubuntu (live CD) to recover data.