Windows and security

I just read a news summary on a MS Vista advertising blurb and it got me to wondering if anyone out there can answer, in a realistic manner, exactly what and how security was improved in each of the past Windows releases. Looking back I can not identify any.

Over the years I have attended many IT shows etc and always visit the MS stand and each new OS claimed to have great improvements in security.

Win NT was twice as secure as Win 3.1
Win NT 4 was 50% more secure than Win NT 3.51
Win 2K was twice as secure as Win NT
Win XP has much more security than Win 2K
And now Win Vista is much more secure than Win XP

In each case they claimed the new one was more secure than anything else.

Now have they been bullshitting all along or was security in past system really that bad?