This is a basic way to get into windows XP without a password (normally) incase your sister changes your password etc. It is also a good idea to secure this account in an office, etc.
Also this is not generally a good idea if the PC is not yours. Probably Illegal.
Step 1: Start Your PC in Safe Mode
When you turn on your PC tap F8 as it is starting, and before it gets to the XP loading screen it should bring up a list of start options (normal, safe mode, safe mode with networking, last known good configuration etc). Pick Safe mode.
Step 2: When You're in Safe Mode.
In safemode there is normally an account called administrator. This is on XP when there is the welcome screen. Click it. It will normally enter with no password.
Step 3: Secure Your PC
When you're in go to "control panel" then "user accounts" and add a password. Also tick the documents private button if you want to be extra safe. It pops up when making a password. This is only to be used for securtiy purposes and it is not my intention for you to do anything irresponsible with this info. I do not like hackers I don't, I do not like them Sam, I note.
So this is a good thing for offices. P.S. sorry for the lack of graphics.
Step 4: Done
You are now done!