depends on the source of the windows installation. did you install from a legit installation source and used a legitimate, purchased serial yet get the non genuine message? if yes, then contact microsoft customer support and ask their help. check ms website too.
if you installed from pirated source and/or used black listed keys... buy a legit copy of windows. you won't find any help with pirated softwares on this forum. pirated softwares themselves are root of almost all software problems. best is to get rid of them and use legit stuff.
you can also look into using linux. linux distros e.g. ubuntu, mint, mandriva etc. are free, no hassle with serials or keys.