When did MS start to give compilers away for free (aka Express editions ?)

I don't have exact time-lines, nor can I think of any easy way of obtaining them, but I think that you're asking the wrong question. The question should be: "When was the horribly broken, automated download of nmake15 added to cpan.pm?"

On the basis of my memory only, but I am very confident that I had been using one of the MS free compilers for building XS extensions for a year or two before I installed a new version of (AS) Perl that suddenly decided to pull and use nmake15 each time I tried to use the CPAN shell. Despite that I had a fully working setup on my system, and nmake.exe was availble via my path; just because it "couldn't find it" by looking in an arbitrary list o wrong places.

My from-memory guess is that this happened around 5.8.6 or later.