USBasp is an open source USB in-circuit programmer for Atmel AVR microcontrollers. The last time the USBasp website was updated, no support was available in Windows 7 x64. This was due to Windows 7 x64 requiring a digital signature for all drivers.
Things have since changed, and the most recent build of libusb-win32 runs flawlessly on Windows 7 x64. This enables the USBasp to communicate with the computer.
You can then program your microcontroller from the commandline, or alternatively use avrdude-GUI as an interface.