Question: I was given by my work an old Dell Optiplex. I did ask the administrator what the password was but he can't remember. when you turn the machine on you eventually arrive at a screen which asks you to give ctrl-alt-delete which then results in the log on screen.
The details (user/department name) are now irrelevant and the network on which it would log on are changed too/no longer in existance. I would like to use this old machine as a standalone for simple tasks, but can't get in. I've tried to find administrator password via ophcrack but couldn't see anything. I can boot up the machine with something other than windows (linux via cd). where would I be able to change settings, which file, or even ditch it and then boot into xp again with the possibility to set up user accounts etc from windows. your help would be very much appreciated

Answer: 
Your best option is to reinstall Windows from scratch. It sounds like they didn't wipe the drive clean, so even if you were able to get into it, you'd still have remnants of the old system in there (Installed programs, registry keys, etc.)
If you were to reinstall Windows, using a license key on a sticker which should be somewhere on the box, than it would be fresh and only have what you need/want on it.

It would also bypass your need to try and crack the account.
It's set up to connect to a domain. Press `Ctrl`+`Delete` to select more options. Select to log onto the machine, not the domain; try to login as “administrator” with no password. Or ask the domain administrator to login in with the domain admin details. This should automatically be set to have admin rights. You can then add a new (local) user with admin access.