Sluggish Inspiron 1520

I'm using my MIL's old Dell Inspiron 1520 after my year and a half old M5010 died. It has an Intel 1525 1.5 Ghz processor, 4 GB of memory (recently upgraded from 2 GB), and a 150 GB hard drive. I did a clean install of Windows 7 Professional 64-bit on it, so there's no bloatware.

The problem I'm having is that the computer is really sluggish when browsing the web. There always hesitation when switching between tabs, and panning and moving around Google Street View is out of the question. I've used Internet Explorer, Firefox, Waterfox, and Chrome, and all of them are just as sluggish. I only have the browser open, so I'm not trying to do a lot at once. Everything else on the computer seems to be plenty quick. It boots up quickly, opens programs fine, etc. Our internet connection is 50 down and 20 up, so I'm getting plenty of speed there. Plus, my wife's new computer has no issues.

One thing I've noticed is the hard drive on my Inspiron is extremely noisy. I don't believe I've ever heard one so loud, however, all the onboard diagnostics, along with HD Tune, say the hard drive is fine. Could the noisy hard drive be related to the sluggishness I'm having? I've disabled the Aero theme, which had a moderate effect on it. Also, upgrading the memory from 2 to 4 GB only seemed to have a moderate effect on the sluggishness. It's recognizing all 4 GB of memory. Is this just a case of the processor being too slow for modern web browsing?

Internet speeds are fine. I'm consistently getting 50 Mbps download speeds and 20 Mbps upload speeds. Both Flash and Java are the latest versions. No browser add-ons or plug-ins. Backgrounds processes area clean and no bloatware since I'm running a clean Windows 7 install. I'm running Avast Free for antivirus. How do I ensure the disk controller is set to DMA? I looked in device manager and didn't see anything labeled disk controller. Thanks for your help.
 

I just found two ATA Channel 0 entries and an ATA Channel 2 entry under IDE ATA/ATAPI controllers in device manager. Both Channel 0 entries have the DMA box checked under advanced setting in properties. Channel 2 does not have a DMA checkbox.

I removed Avast altogether, with no change in performance. Could a hard drive be going bad even though diagnostics say it's ok? It seems to be the most sluggish when the hard drive is reading/writing a lot. I can actually hear it from a good twenty feet away, with the TV on, to give you an idea of how loud it is.
 

I've run chkdsk multiple time with no errors. I'll try browsing in safe mode when I get off work this evening and post the results.
 

I ran chkdsk again and it returned no errors. Safe mode didn't seem to make much of a difference. Below are the processes that normally are running:

Did the above with no change in performance. Even scrolling through the message list on the forum is sluggish with a delay from the time I move the scroll wheel to the time the screen starts scrolling if I have another tab open and loading a site.