Since the build system for windows doesn't work anymore I updated the compiler and tools in it. Many others have same issues building Xonotic (here, here and here) so I decided to fix it and it's working for me (Windows 7 64-bit).
Just download the archive and unpack it somewhere, run build.bat and wait for it to download and compile the game like in the old build system.
If you don't want to redownload everything you can merge the new one with the old one and replace every old file. Alternatively you can unpack it in a seperate folder and copy the "xonotic" folder from your old build system to the new folder. But if you don't redownload everything you have to do "./all clean". Just run shell.bat and enter cd /xonotic (press enter) and ./all clean (press enter).
I hope it's helpful
Download 7z archive
Just download the archive and unpack it somewhere, run build.bat and wait for it to download and compile the game like in the old build system.
If you don't want to redownload everything you can merge the new one with the old one and replace every old file. Alternatively you can unpack it in a seperate folder and copy the "xonotic" folder from your old build system to the new folder. But if you don't redownload everything you have to do "./all clean". Just run shell.bat and enter cd /xonotic (press enter) and ./all clean (press enter).
I hope it's helpful
Download 7z archive