Hmm, you said your computer has 512mb ram (I thik that's what the 512 in the end of your post means), is it pc133 or rd-ram(rambus)?
I haven't seen any machine with that much rd-ram since it's still very costly, and if it's pc133 ram that could be the reason since the P4 is very dependent on memorybandwith, thus running it with pc133 ram would be like having a fast car but only using the first two gears.
I've heard that a P4 1,7 with pc133 ram actually isn't faster than a 1,0Ghz P3 (also with pc133 ram, but it's designed for lower memorybandwith so it doesn't matter), but I don't know if this exapmle is correct.