My son has an HP laptop and playing around, changed his password and then couldn't get into his account.  There were a number of problems here, any of which we could have fixed had we been paying more attention earlier on (like soon after he got his laptop):

1. We could have created a password recovery disk (which can also be on a USB keyfob) - but we didn't.
2. We could have created a secondary administrative account on the machine with a password known only to me, his dad - but we didn't.

We've now done both of these things since we were able to restore access to the machine under his previous password.  But it was a bit of a trick to get access under the previous password.  I figured we were going to have to reinstall Vista, using the File and Settings transfer program to first grab his old settings and files, and then reinstall all the programs - probably a 3-4 hour undertaking.  But then I had another thought.

The key here was using Vista's System Restore feature. The exact steps will probably vary on your machine (and this is from my memory on his HP laptop, having just done this) but roughly what we did was:

1. Harangue him for a few hours to try to remember his password, try different variations (with and without caps lock on, for instance) and try omitting characters he might have thought he typed when he changed the password but didn't.  That didn't work. :)
2. Reboot the machine and press F11 while it's booting - on HP machines, this gets into the "recovery console" which is thoughfully put on the system disk in a separate partition by HP.
3. Select to restore the system (note: not restore to it's original factory condition - that will reformat your main partition and wipe out your files, which we really wanted to avoid).
4. Chose to reset to an early restore point - in his case, there was a restore point just four days old.  Vista creates these restore points regularly.  The only thing he had done in the last four days was reset his password, so I thought there was a chance that Vista might reset the old password along with the restore point.  I wasn't sure it would, but I thought it was worth a try.
5. It took about ten minutes for Vista to do this and it then rebooted - and he was thrilled that he was able to log in with his old password!

I've used System Restore in other cases to get rid of really wierd behavior, and it always works like a charm. This is yet another use.

Oh, yeah, and we then created a secondary administrative account on the machine.  Do that right now if you have a laptop with just one admin account - then squirrel away the password somewhere safe at home and you'll thank yourself one day.