you will have to restart the computer, and use either a password-override cd to get around the block, or another option is to restart and use a windows operating system cd to install a fresh copy of windows. Whichever you decide, you need to boot from the cd at system startup by pressing either F2, F8 or F12 - these are usually the keys used; it is only one of these - and boot from cd.
If you want to reset the password ONLY, then try the following links where it will tell you what to do and what to download from the selection given.
http://pcsupport.about.com/od/toolsofthetrade/tp/passrecovery.htm
Hope this helps!
Mike 'Cocopenguin' Wing, October 2012