performance - Why won't Windows use the other CPU cores?

In Windows Task Manager the Performance tab shows the first CPU maxed out, the other 7 just idling along with the occasional spike. What gives?

More info:
I've got 8GB and only 4.5GB are being used. The Processes tab has no indication of any process hogging processing power. In fact System Idle Process is 98-99.

When I program stuff and have like 8 to 12 applications going (several directly unrelated to programming of course) my computer slows to a crawl.

System Info:

Intel Core i7-2600K Processor (quad-core with hyper-threading), 
8GB RAM, 
Intel BOXDZ68BC LGA 1155 Motherboard, 
500GB HDD

Answer

Many apps don't use all cores. They aren't programmed for it. Then they will use one core and will overload it.

But you can tell Windows to run that process on 2,3, ... and in all cores.

For doing it, follow these steps:

1. Open Task Manager


2. Select tab Processes


3. Right click and select 'Set Affinity'


4. Select on which cores you want to run process.