It's not "suddenly". GZDoom's 1.4 branch was that of the last great GL code rewrite. There have been many added features, and especially relevant here is the fact that it queries the driver to know whether certain extensions are available. For some reason, many driver writers think it's clever to have the driver say "yes, I do support that whatever you said" when it's not the case, and then you get a crash as the extension is attempted to be used.
As for Skulltag, the 97c branch used ZDoomGL and the 97d branch used an old version of GZDoom.
Really, it's not GZDoom (it works fine on other machines, including with Windows 7), it's not Windows 7 (it works fine with GZDoom on other machines), so it's either your hardware that's faulty or your drivers that are bad.
Since you have an ATI...