On the welcome screen press CTRL+ALT+DEL twice. Type username as Adminstrator and leave the password blank and press enter. If you're lucky then you'll enter the system. Lucky in the sense that, if the system owner didn't enter an adminstrator password while installing windows then you're lucky.
If you're'nt lucky according to the previous trick then this trick will forcefully let you enter inside! It works for sure. You need another computer with internet connection, a cd writer and a blank cd off course.
1. Download this file reset.zip
2. Extract the zip file to get an iso image.
3. Burn the iso image on a blank cd.
4. Boot the target computer with this CD. Follow the steps on screen and reset the password as you wish!!!
This trick works for resetting Vista too!
If you've access to the target computer with an account with administrator privilleges then you may use this command on command prompt to reset any user's pass
net user <username> <new password>
net user administrator newpass