The other browsers are right; IE6 is wrong.
The selector a should match any <a> elements, while a:link only matches <a> elements that are unvisited hyperlinks (the HTML 4 document type defines hyperlinks as <a> elements with a href attribute). Nowhere does it state in either specification that a should automatically translate to a:link or vice versa.
Since there's no such translation going on, your two CSS rules have equally specific selectors (your class selector shares equal specificity with each of your pseudo-classes). So, your second rule is supposed to override the first rule for any <a> elements within div.myclass, regardless of their link state, thereby making it always red and with no text decoration.
By the way, IE7 also fails to apply the font-weight: bold style when you test with an <a> element in div.myclass that isn't a link, even though it's supposed to as there is no overriding font-weight style in your second rule:
<div class="myclass">
<p>This is a <a href="2">test</a></p>
<p>This is a <a>test</a></p> <!-- does not bold on hover in IE7! -->
</div>
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