is a very sensible question, and I can see why you ask.
On the one hand I sympathise with your sense of having been deprived of something, yet on the other hand I sense your feeling that there may be a rocky road ahead. That isn't to say I'm advising against any action - merely that my experience tells me it might prove difficult.
'Caveat emptor' is an oft-used phrase, but it does have great relevance in some situations, and this is one of them. The law takes the view that it's every sensible person's duty to make sure that what he/she is buying is what he/she wants, and to exercise common sense when approaching a buying situation. It sounds as if you did that - you certainly remember seeing the card reader in the display machine, and presumably you knew what other components/software would be included in the purchase price. In such a case you would be said to have 'relied' on the description of the machine when making the purchasing decision, and the computer would be 'not as described' if the specification was changed in any material way.
The problem comes when you subsequently try to prove that the display description was as you say - especially when it's been removed. It's now your word against the store's, and that's why I sounded a note of caution above - they've already told you that the display model didn't have a card reader. It's now your word against theirs, and that's never a happy situation. The online machine image is interesting, and I would certainly mention it, but it's that in-store display machine that is the clincher.
In the final analysis you have three choices as I see it:
1. Contact Comet and say that the computer isn't the same as the one on the display (the one on which you based your decision to purchase), and ask for one with a card reader.
2. Tell them that the computer isn't as described and you want a full refund.
3. Do nothing.
It sounds as if you don't intend to settle for option 3. and I imagine you'll be on the phone tomorrow.