That guide is a waste of time. You do not want all of those un-needed programs, it will clog your computer.
Ok your best thing is to clean out all of the dust inside of the computer, make sure the heatsink fins are clear of any debris.
The easiest thing to do is just do a complete reinstall of windows, it will run like new! Also after you get the new reinstall, you should put a password on it so your sister cannot get on.
Now you should defrag your HD around every week. Or after you install/delete alot of programs.
to keep the startup nice and tidy if you should goto
START> RUN> msconfig> STARTUP> uncheck all of the unnesecary items that you do not want to startup automatically.
Also A/V is needed, and a 3rd party browser ( Mozilla Firefox, Opera )