Hi Singo30,
If the PC shipped with Windows 7, you shouldn't need to use the Windows product key. The key should already be integrated into the preinstalled software.
Just for reference, here is a link to your product specifications:
http://h20565.www2.hp.com/portal/site/hpsc/template.PAGE/public/psi/learnUseDisplay/?sp4ts.oid=5270849&spf_p.tpst=psiContentDisplay&spf_p.prp_psiContentDisplay=wsrp-navigationalState%3DdocId%253Demr_na-c03364240-2%257CdocLocale%253Den_US&javax.portlet.begCacheTok=com.vignette.cachetoken&javax.portlet.endCacheTok=com.vignette.cachetoken