My lap top wont startup its saying that windows 7 has to be installed again

What kind of laptop is it? Dell used to use an image patition to hold a disk image of the OS in out-of-the-box condition, which you could restore using a key-sequence at boot - you'll have to google it, don't make me do all the work :~)

Laptops that came with os usually had backup/restore partition to reset the laptop to factory default setting (including the OS)

Theres a certain key need to be pressed at Post screen as trasher32 said..

but i hope u had backup, restoring usually destroy/clear the os partition..