hey i have lost my windows 8.1 cd and now want to format and clean reinstall it because of some malware.

i have the genuine product key with me but can you guys help me on how to reinstall it with no windows installation dvd ?

Here you go follow the link here:http://windows.microsoft.com/en-CA/windows-8/create-reset-refresh-media

They will show you how to make a media of windows 8.1 after that all you need is your key

You can use the Web Installer:

You can download it from there and use your product key to upgrade:

http://windows.microsoft.com/en-GB/windows-8/upgrade-product-key-only

You can also use the Web installer to create a bootable thumb drive:

http://techingiteasy.wordpress.com/2012/11/07/part-2-how-to-upgrade-using-the-windows-8-upgrade-assistant/