Mathematica - how to read in a number in hex and convert it to binary?

Hi, newbie question I'm afraid, but I couldn't work out how to do this from the help. I'm using Mathematica 5.2.

I've got a textfile containing a long list of numbers in hex. I can read them in one by one, using e.g.

p=Read["hello.txt",Expression]

Then I would like to turn each number into a fixed-length binary number, using something like

IntegerDigits[16^^p, 2, 27]

The problem is, this doesn't work. The argument of ^^ seems to need to be an actual number in hex, e.g. 16^A12B3, rather than a variable.

Will be grateful for any help with this.

Thanks!

Michael