Well i tap both ALT+F10 while booting my acer laptop,
press enter at the "edit boot options" screen
select "restore to factory defaults"
The good thing was i did'nt lose any files which i saved into my imation,
i also learnt not to messed up my settings.
The best part was i did'nt need any dvd to restore my windows7.
Thanks to everyone for your replies and advices.