Computer responsiveness issue

When I use my computer to download anything over 100mb or such my pc slows down. I mean that it takes forever to install a 5mb file, for this example we will say a mass effect 2 armor file. Short and normally quick, but not when I have a download going this 8mb file is normally under 5 seconds to cycle all the way thru the installer. With a download going its up to 1 minute or more to cycle thru the installer. Also defragging is extremely slow eventhough it is 100% offline and overall the pc is unresponsive with a ton of mouse lag. Finally if I launch an offline game, while anything is downloading it slows the game to a crawl which makes no sense due to the fact that the game witcher 2 has nothing to do with online. I am operating in 100% single player/offline and steam is in 100% offline mode.

For some reason its always been this way, but I would love to find a fix for this.
I understand it if I was using programs in multiplayer or networking, but I am not.

PC Stats
Q6600 @3.2Ghz 1.29v Linked/Synced 1422QDR, 711Mhz 4-4-4-12-2T
4GB PC 8500 Dominators 2.100v
E-vga 680i SLI A1 with P31 Bios and E-VGA 260 GTX 216 896MB
Windows 7 Home Premium SP1
2x 500GB Seagate's 7200 RPM

Yesterday I completely wiped my system due to no internet connection issue where I couldn't release a 169.X.X.X IP address.

I installed Avast, IDM, and MBAM.
I disabled windows defender, because it has always made my pc slow. Disable the realtime protection of defender and you'll see a performance improvement immediately. Ran a scan after all windows updates were in place just incase.

Hi, a few questions:

- does this problem occur again now that you have done a clean install?

- you can use Process Explorer (from Sysinternals, now part of Microsoft) to check which process is using up CPU and slowing things down

- is it only during download or during any intensive disk activity? if the former: do you have the latest network drivers?

- if the latter: are your disks ok? Can you do a "chkdsk" and also run Seatools utility to check SMART status (look for the reallocated sector count) - too many errors means the system will be really slow

- are you disks connected as AHCI - this will improve disk access performance

Hope this helps you to get more information, which you can use to debug further.