I hd a closer look at this today, and I'm 99.97% sure it's a false positive.
I had a look at it on the Virus Total website, which does report some of the exes and dlls contained in the zip to be suspicious, but only by a small number of virus checkers (it' s a 'meta' checker) and all weird ones I'd never header of, none of the 'big name' ones.
So I went and rebuilt the entire Blitz3D packge from scratch using slightly different compiler flags to see if that made a difference and checked it again, and it did! I got a totally different results with fewer false positives (still not 0 though). So it's almost tempting to 're-release' this version as a fix, but I wont be doing that as I don't want to have to be effectively 'randomizing' compiler settings for each release to suppress virus checkers, that's just nuts for a whole lotta reasons.
I guess there's a chance that my machine has been infected by something that infects exes I build using MSVC, but short of reinstalling the whole OS (and I'm not even sure that's enough any more) I don't know if there's anything I can do about that. My machine is regularly checked by MS Windows Defender and I recently checked everything with Bitkeeper for variety which did quarantine a few things I'd never heard of (and similarly have no way of knowing if they were 'real' problems or not) but otherwise said my machine was healthy.