May be this will work for you.
I hope you have more than one drive in your PC.
put all windows seven installation file in any drive except where it is currently installed for ex. if your current windows in drive C\ then copy files in drive D or any other drive but not C.
Now click on setup.exe and start installing windows as normal. It worked for me on Vista a long time back but i am not sure about seven so you better be careful.
And there is one more option install it in some different drive like if currently installed in C\ drive than install it in driver D. After installation format the drive C\ from drive D.
Both solution worked for me on Vista like 2 years back as i did not have the bootable DVD.
But i never tried it on seven. SO think twice and do some R&D on it before using any of these options