Normaly, there is a key combo to press as you power up. Check your documentation.
This will destroy any documents, remove any installed application and put back the crapy and obsolete trial wares the computer came preloaded with.
It may not be possible though: If the main drive have been replaced, or repartitioned to get more storage space, the factory restore image and utilities are suppressed.
As the restoring to factory settings is VERY destructive, you are probably beter to use a normal restore point.
If you realy need to whipe everything clean, I suggest reformating and installing Linux.
Electro, March 2013