new hardware in slow computer points to my HDD but not sure

previously, i had these specs:
e8400 C0 @3ghz
780i SLI
8800gtx
7200.11 32mb 500gb
A-data 800mhz 2x1gb

I was experiencing slow speed despite these specs; it was slow just opening up Visual Studio 08, browsing around the computer, and opening up word documents, for example. The BIOS was up to date and so were the motherboard drivers. No viruses, removed the HDD jumper, etc. A fresh windows install yielded no improvement.

then i started eliminating hardware as possible causes. These are my new specs:

*new e8400 E0 @3ghz
*new Asus P5Q Pro
8800GTX
7200.11 32mb 500gb
*new G.Skill 2x1gb 1066mhz

BIOS and motherboard drivers are up to date.

I know, after all that it was probably my hard drive. but long before I started the process of elimination, i bought a new HDD, same brand and model. No improvement.

The interesting thing is, at first i had in the build a 7200.10 8mb 160gb HDD. Same problem: slowness. I fooled around (dont remember exactly what i did or else i wouldnt be here with this), installed some utilities for the HDD like HDDtach and HDDtune. at last i updated the bios for the first time and my computer got blazing fast. this was when my new 7200.11 was on order. I figured id just pop it in and itll be even faster, as it seemed that the bios was the problem the whole time. but as soon as i got the new hdd up and running, it was slow, much like what i experienced with the 7200.10. very saddening, let alone aggravating. I was never able to figure out what happened.

as crazy as it sounds, this leads me to believe that something i installed on my 7200.10 (the one that i got blazing fast) rid my computer of the slowness. I could have reworded that.
What are your guy's thoughts on all this?