I think what you are trying to ask is how to reinstall the operation system ..
the issue here is the lack of disk drive ..and there are 2 ways to reinstall an operation system ..One ..is to download an image with the operation system ..and extract it from the archive ..into drive D ..and run it from there..This will suposed to already have the capability to boot into windows and run the installation over it ..
The second and the one I recommend is to install from a flash drive usb made boot-able with the operation system on it ..
To do that you need a flash drive ..at least 4G ..and a program to make it boot-able and load the files on it ..
here is the program .you need ..(it works perfect with windows 7 and in windows 7 ...if you use a windows xp to make the usb stick boot-able ..you need several Microsoft updates to be installed first ..http://download.cnet.com/Windows-7-USB-DVD-Download-Tool/3000-18513_4-10972600.html
and you also need an image with windows 7 to be downloaded into your hard disk ..and after that choose it to load it into the usb memory device with the tool I linked..
here is a choice (you can use a windows disk to make the usb , if you have one )
use a torrent client and download this image into your computer http://thepiratebay.org/torrent/6780029/_Windows_7_Ultimate_32-bit_Original_iso_Team_Rjaa_
if you don't have a torrent client (bit torrent or other ) use utorrent cause it is non invasive and reliable and tiny ..http://www.utorrent.com/
to download the image with windows ..click on download this torrent .., avoid that big download button cause it is a spam ..
OK ..lets take it from the start ..assuming you do not have but this net-book you want to install and a usb 4 G
download the utorrent ...and the tool for making the usb and run them into computer ..
after that go to the link for windows..and click download this torrent
it will start the download with torrent automatically ..and will not be too fast ..but take about an hour or so ..
when it is finished..close u torrent (remove the torrent first from teh list (select and click remove )
Now you have the image for windows 7 and the usb stick and the tool ..open the tool for loading it to usb ..and select the flash drive by browsing for i t..than next ..You have to select the image with windows..Browse and select ..and next ..Now will ask to confirm the formatting of the usb ..and after that will load files..When it is finished.. Close the tool ..and you have a boot-able usb with windows ultimate on it ..
save all files you may need into other partition ..and also if you do not have ..download all drivers fro the support site ..and restart computer with he stick on .
press f12 or f11 on start (it is specified on the boot screen what to press for boot menu ) ..and when the boot menu is on ..select the usb flash to boot from ..and ok ..It will start windows installation and the only intervention you have to make is to select teh C drive and format it (using advanced tool for partition ..and format it quick ) ..after that will load by itself and restart several times during it ..
When it is ready ..it needs all the drivers for the devices..this is why I suggested to load them in D drive before so it will be easier when you get to this point ..you also need a antivirus ..and right after the connection to the internet is made I suggest to use the Microsoft essentials ..cause it is free and it works great with windows 7..
Please do not hesitate to ask if you have problems ...By the way ..If you have issues to boot from the usb device ..you can start the windows installation direct from windows..just open the flash drive folder and click on the set-up ..and will start from windows and will go on after restart ..
Please rate my answer cause it helps my activity here ..and let a comment if you need any further assistance ..
have a nice day !