As std76 said the parent will need position:relative to create a stacking context but in IE7 you will also need to ensure the element is in haslayout mode or it won’t know where its boundaries are and misplace the element.
Just add zoom:1.0 to the parent (or if you need valid code use one of the other haslayout triggers (see css faq)).
However, without the full code we might both be guessing at the worng thing