1) Hope you learn from your mistakes for the next time. Write the password down and put it in a safe place... and remember where you put it. Or better yet, use a password that makes sense to you... Or the best of all, create a Password Reset Disk
(details abou this here: http://support.microsoft.com/kb/321305
2) The "brute force" alternative: Re-install Windows from its installation CD/DVD (which I hope you didn't lose too). This will erase all programs and files you have in your computer. You'll need your Product Key (you have this, right). You may want to try to do an "update previous instalation" (which will keep your programs and files), but it may not remove the password at all.
3) Get a password recovery tool (for free, if possible). Many Windows versions (especially XP) do not like if you just 'delete' the password from an account, like some programs can do. If you do this you may lose access to many files and programs. This a security feature embedded in Windows.a) Ophcrack:
This Windows password cracker is by far the best free Windows password recovery tool available. It's fast and easy enough for a first time password cracker with a basic knowledge of Windows.
With Ophcrack, you don't need any access to Windows to be able to recover your lost passwords. Simply visit the site, download the free ISO image, burn it to a CD and boot from the CD. The Ophcrack program starts, locates the Windows user accounts, and proceeds to recover (crack) the passwords - all automatically (access to files and programs will be kept, so you will regain full access to the computer
In a test on a Windows 7 PC, Ophcrack recovered the 10-character password to an administrator account in 40 seconds. Ophcrack supports Windows 7, Windows Vista, and Windows XP.Download Ophcrack from here (choose the correct OS): http://ophcrack.sourceforge.net/download.php?type=livecdb) Offline NT Password & Registry Editor:
This one works basically by erasing your Windows password instead of recovering it (ths is where you can lose access to some files and programs). You can then simply log in to your account without entering a password.
Like Ophcrack, you boot to a burned CD created with the Offline NT Password & Registry Editor ISO file.
If you like this "password deleting" strategy then I highly recommend this program. Offline NT Password & Registry Editor works basically the same way as PC Login Now, but this program is free.
I tried Offline NT Password & Registry Editor on a Windows XP PC and it worked immediately without problem. It should work equally well with Windows 7, Windows Vista, Windows 2000, and Windows NT.
There are some other utilities out there, but finding FREE ones that actually work is time consuming. I'll post them here as I can.