If you had to spend let's say between $500. and $600.for a good new desktop (pc),which one would you buy? It seems that,for the same criterias, HP is $100 more expensive than Compaq, is it worth it?

HP and Compaq are one company... I find it hard to think that the exact same units with identical equipment would be $100 more than the other... Not good business....
Was one unit on sale ? I recently brought a new Compaq for about a hundred less of a HP because it was a closeout on a spacific model...If thats the case, by all means buy the lesser priced one... Just check and compare the two models side by side for equipment...