If the PC was supplied with win7 preinstalled either it has a hidden recovery partition, which you access via that PC makers instructions to create the recovery disks, or it was supplied with a set of recovery disks.
How you restore to factory condition is a process determined by your PC maker, its not an MS or win process.
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This is possible only if your PC maker provided the facility to do so [by putting a Factory Image Restore partition on the hard drive]. You will need to check your PC manual or the PC maker's support site to find out.
The PC maker must have provided a means of reinstalling Windows 7 - it is only the chosen method that can be in doubt.