Fresh download of http://download.qt-project.org/snapshots/qt/5.1/5.1.0-rc1/backups/2013-06-14-57/qt-windows-opensource-5.1.0-rc1-mingw48_opengl-x86-offline-2013-06-14-57.exe on a Linux system. copied on USB stick.

Installed from this USB Stick on Win XP with Avira, and while the installer installs, Avira already alerts about the Qt5WebKit.dll in the installer being infected with TR/Crypt.ZPACK.Gen2.

Thus the infection cannot come from the local system. Whatever Avira doesn't like, it's in the installer already. Either a false positive by Avira or a real Infection, I dunno.

Please see https://bugreports.qt-project.org/browse/QTBUG-31531 - I commented there but it's closed and I don't know how to reopen. (Merge the bugs if you like.)