If you have no password reset disk, according to Microsoft, you are out of luck and would have to wipe the disk and reinstall.
There is an unsupported method... If you do a search, there are plenty of Rescue tools (some free and some paid) online that you can use to boot off of a DVD or USB key to access the local account manager on the Windows box and reset the password. These tools are basically Linux live type of instances where you are basically booting to a version of linux with a bunch of tools at your disposal. AGain, you do this at your own risk. Its not supported by Microsoft.
I always wonder in these cases whether the OP is the legitimate owner of the login... but this stuff is all over the web, and the real owner would likely prefer to have his system whole and workable, if indeed pryed into. A sys in someone- else's hands is a lost system. So... here goes:
Jorge is quite circumspect when it comes to issues delving deep into the heart of a system, and probably quite rightly (I suspect that he is a sysadmin).
But it's your system, your risk, so I shall tell you what works for me and W7 (Pro). Just be aware that if you employ Windows EFS file encryption then at some time you WILL need to recall your password to access encrypted files.
However, I have used this password reset disk on XP and W7 to no detriment. http://pogostick.net/~pnh/ntpasswd/bootdisk.html is the official site to download from. I have used vsn 110511 (that is the release date as well as the name; it just works, so I have not bothered to update). There is a later version on the page.
Download, burn the iso or install to USB. Boot from that drive; mostly the answers are the default, but read the questions! If you enter your account name when requested in the edit section, then case is important.
Advice is to clear, and not to change the password with the software. Save the edit and restart.
do u have another account if u do go to it and open run(window+r) and type lusrmgr.msc when it opens a new window go to users &right click on what u want and say "SET PASSWORD ".it will not ask u about the old password.