Slow computer when reading/writing

Noob here. I tried posting about this some time back on an AMD forum but didn't get anywhere.

About a year ago I upgraded my motherboard/processor from a Gigabyte/5600 to my current MSI DKA790GX/Phenom 9950. All was well with the previous package. This new combo has had a problem from day one, and I've finally had enough.

Whenever there is any sustained drive activity, the foreground app slows to a crawl. Here's a typical example... I'm downloading a large set of compressed files (movies) using Forte Agent, and if I try to use my browser, or start another program, it takes so long I might as well just wait 'til Agent is done. Pages that normally load in a second or two with my browser, now take 20-30 seconds, sometimes much longer. You'd think downloading files wouldn't put any strain on the system, but it basically stops everything else. There was never a problem like this whatsoever with the Gigabyte board (using the same drives).

Another place this is noticeable is when scrolling in my file manager. If any program is doing a lot of reading or writing and I drag the scroll bar down, it starts hiccuping the line display, similar to if I had a bad video driver. Basically, the drive is being hogged by the background app and the filemanager cant' read fast enough to keep up with the display.

I've tried different drives (Seagate - Samsung SATA's) and different ram. Nothing has helped. I currently have two sticks of 2gb and two sticks of 1gb ram. Nothing is overclocked. All stock.

I've installed the latest bios and that didn't help. I defrag the hell out of the drives. I've tried an empty drive with virtually nothing but the operating system... Nothing.

I'm thinking something in a bus/driver?

Any help would be greatly appreciated.