Lock computer in XP

ctrl -- alt -- del -- choose Lock Workstation

you can press ctrl, alt, & delete at the same time and select the option to lock workstation, or you can right click on your desktop, go to properties, select the Screen Saver tab, choose a screen saver and check the "password protected" box and set up how long you would like the computer to wait when there is no activity to lock itself.

If you're trying to lock out the person with the second profile and they have that password all they have to do is reboot. If they don't know the password follow the instructions of the other posts.

If its a situation where you don't want the second person using the PC when your not around like in the case with a kid just shut down the PC disconnect the keyboard and mouse and hide them. Don't unplug with the power on.

Right click on your desktop and select New/Shortcut. In the location box type EXACTLY:
rundll32 user32.dll,LockWorkStation
Name the shortcut whatever you want. If you put the shortcut on the taskbar you can lock the machine with a single click. Do a search of your hard drive for: security.ico and you can change the icon to a nifty lock and key.

Due to my constant persuit of efficiency (laziness)using a minimum of 3 fingers, cntl-alt-del and then selecting to lock the account can be time consuming (exhausting!!!) I find that using the 2 finger method of the Windows Key & lower case L, is more conducive to saving time and increasing preformance (slacktime!)

You could use a key I use this device http://www.forum.ledsystems.co.uk/forum_topics.asp?FID=45

From your question it appears that the computer is not part of a domain and you have 'Fast User switching'turn on. That means you will have to do the following.

1. Set a password for your account. Once the password for your account is set...

2. To lock the computer from other users accessing your profile press WinKey+L. A window will appear showing the user accounts. For your profile it shows the number of running programs.

3. To unlock the account - when you select your profile you will be prompted to enter a password.