In general Windows 7 is a good bet. But if you run into trouble, PM Webbot on these forums. He's very good at replying and helping.
I've found windows 7 64-bit and webbot studio to work well. However have not been able to workaround compile errors with webbot studio and windows 8.
I've had this issue too, on two or three 64bit systems.
I've managed to work around the issues on Win 8 (And Win 10 preview, just today), and am going to message Webbot about this in the next few days. Quick answer is the following steps:
0) If webbotlibstudio is giving you errors during setup when it's building the files, just keep closing the errors, there's going to be one for each processor....
1a) In the WebbotLibStudio install directory, there is its own copy of WinAVR-20100110. It has a subfolder utils/bin, which contains a few versions of the msys-1.0.dll file. Rename the one with that name to e.g. msys-1.0.dll.orig, and then rename msys-1.0.dll.64 to msys-1.0.dll. This file that you want has a Date Modified of 5/18/2007.
1b) (only if you installed WinAVR separately from WebbotlibStudio) In the install directory for WinAVR-20100110, again in utils/bin, you want the msys-1.0.dll that has the 2007 timestamp. This probably doesn't exist, so rename the existing one, and copy the one with the 2007 timestamp from WebbotLibStudio's copy of WinAVR to this location as well. (Also, WinAVR-20100110/bin needs to be in your PATH variable, but I'm assuming that was taken care of already...)
2) Since webbotlibstudio presumably failed to build the avr libraries (aka step 0 above), we can build them ourselves. If you navigate to C:\*WebbotInstallFolder*\WebbotLib\AVR8 in command prompt (where the last part of *WebbotInstallFolder* is probably WebbotStudio-1-03b), and then run `make clean` and then `make all`, you will repeat what failed in step 0.
Note: Step 1b needs to be done if you ever build AVR code outside of webbotlib studio. On Windows 8, step 1a allowed step 2 to work. On my Windows 10 setup, I had to do 1b to get step 2 to work. This is probably because I have the PATH variable set differently on the two computers; I haven't had a chance to double check that this is indeed the case, though.