WinAVR (h**p://winavr.sourceforge.net) is a good option i think. I haven't tried it yet, but it seems to be able to be integrated to AVR Studio. Good thing
I use WinAVR on a daily basis , and i like it.
The AVR-libc guyzz just released a new avr-libc , and it will hopefully come into the new version of WinAVR.
If you go for the latest release of WinAVR (AVR-GCC) for windows , then make sure to pop by the GCC forum at www.avrfreaks.net , there are some corrections.