hi i am using an acer aspire4736 model laptop.unknowingly i have cleared registryfiles with tuneup software.From then my system is not loading o.s.just it is showing boot options then a blank screen is appearing and again boot options like that.and i tried to boot it from pendrive but it is showing the option to boot from pendrive but i was unable to boot.then i tried with a DVD then it is not showing any option to boot from a DVD.please help me.