We have lost the administrator password of a Windows Server 2003 machine which is used in our lab. Is there any way to reset or recover the password without reinstalling Windows?
There is a linux live CD that contains an utility for changing the password. I managed to reset my lost administrator password that way. Here is the link: http://pogostick.net/~pnh/ntpasswd/
Edit: if the server is a domain controller (i.e. you are not able to log in to "local machine") you can always login as "local" admin in domain recovery mode (accesible by pressing F8 during startup), there is a step-by-step guide how to reset W2k3 domain admin password here: http://www.petri.co.il/reset_domain_admin_password_in_windows_server_2003_ad.htm
OPHcrack is a good shout depending on how complex your password was
also you can try this can be a bit messy tho
If the machine is on a domain, login to it with an account that is in the Domain Admin group then you should be able to reset the local admin password.
If it isn't on a domain (or there is something locking a domain admin from changing the local admin password) then one of the many boot disks that can reset NT password (see other people's replies for links) will do the trick - I've used similar tools successfully in the past.
If there is anything important on the machine then I suggest that you try back it up before messing around, just for paranoia's sake in case the password resetter fails in a way that makes things worse. If you boot from any good Linux Live CD you should be able to mount the drive nd read the files, and mount a network share to copy important ones to (or you could take a full image of the drive, but that is almost certainly overkill).
Instead of booting to a Linux Live CD and hacking at it, you can use a commercial product like "Active Password Changer" (http://www.password-changer.com/) that costs $50 and it will make it super easy for you do view or reset any of the windows passwords on any machine. Its almost ridiculously easy to use.