Have a look at this first
Reset User Account Password
From what you say , you may need to do this
D/l this .iso
PCLoginNow, Download PCLoginNow
burn to cd with this - do not format cd-if autoplay pops up, just close it.
Boot from the cd. It finds your o/s-highlight it and click Next. It finds the accounts - highlight the Admin account and click Next. Tick "Password is empty" click Next - it asks if you want to do anything else, say No.
Remove the cd. Reboot the pc.
Hope it helps
EDIT: How to boot from cd/dvd :
You may find you have the option of a pop-up boot menu - watch the black screen with white writing when you start the machine - may say something like:
F2 for setup
F11 for boot menu ( your F numbers may be different)
Press the one for boot menu - when it pops up, arrow to cdrom drive , put dvd in press Enter. It will ask you to press any key to boot from cdrom - press Enter again.
If you don't get that option , see this
How to Set BIOS to Boot from CDROM - www.hiren.info