if ur using vista or win 7 (32 (x86) or 64 bit)
then no need of pendrive.
download magic iso , install it
get an image of win vista or win 7 in .iso format i.e extension of image file should be .iso and copy it in any of ur drive exce[pt ur windows drive
now right click on the .iso file, ul get an option of magic iso,go to it, there ul find an option mount image, click on it
open my computer, ul find a cd drive showing ur windows cd (which u mounted using magic iso), autoplay it, it wil show install option, go ahead n follow the simple steps.
one option ul find is of copy files to hard disk, use that option and continue, this way ur computwer with carry on with the process and restart to take u to the format option, install the windows,
after installing go to ur windows drive right click on it n go to properties, click on disk cleanup, n remove the windows installation file, rather u can select all the options n clean it.
this free ur wndows drive memory from the setup files, n thus ur done:-)
Let me know