Quoting from this article:
http://support.microsoft.com/?kbid=321305#SafeMode
1. Restart the computer. After the Power On Self Test (POST),
press the F8 key.
2. On the Windows Advanced Options menu, use the ARROW keys
to select Safe Mode, and then press ENTER.
3. When you are prompted to select the operating system to
start, select Microsoft Windows XP edition (where edition is the
edition of Windows XP that is installed), and then press ENTER.
4. On the To begin, click your user name screen, click
Administrator.
5. Type the administrator password, and then press ENTER.
Note In some cases, the Administrator password may be set
to a blank password. In this case, do not type a password before
you press ENTER.
6. Click Yes to acknowledge that Windows is running in safe
mode.
7. Click Start, and then click Control Panel (or point to
Settings, and then click Control Panel).
8. Click User Accounts.
9. Make any necessary changes and reboot your computer.
How to start in Safe Mode:
http://support.microsoft.com/kb/315222