I have uploaded USBasp drivers for windows 7. Drivers can be downloaded from the following link:

https://www.dropbox.com/s/30yl4p8tbrd3qr6/USBasp%20-%20Drivers%20and%20Sources.zip?dl=0

Instructions:

1) Download and extract contents into some folder.
2) Connect USBasp to PC. Go to Device Manager, locate “USPasp”. Right click and select “Update Driver” .
3) Browse for extracted folder and proceed as usual.
4) “Windows Security”  warning will appear. Select “Install this driver software anyway”  and proceed.

Thanks to Mr. Nandan: http://elecrom.wordpress.com/2009/04/19/usbasp-tutorial-usbasp-driver-installation/#comment-2149