It's all of the crap that you accumulate by installing software, loading down the registry, drive becoming fragmented, temp internet files, you name it. I'm sure after you format you'll see very few tasks running in the background, but they'll increase tremendously as you install and use the necessary applications to utilize your computer.
Can't really be fully avoided, you can try to defrag, clean up your registry, manage the windows services, remove programs from startup, etc.
Unless this is a more severe problem than what I am referring to. Care to elaborate?
PC - | Core i7-2600K @ stock | Gigabyte GA-P67A-UD3 | 4GB Patriot Sector 5 DDR3 | Galaxy GTX 460 1GB @ stock | Corsair Force 120GB SSD | Seagate 1TB 7200RPM | 320GB Seagate 7200.10 | 2x Seagate 500GB USB 2.0 | Lite-On 20x SATA DVD+RW | Silverstone TJ-09 | Corsair 620W PSU | Logitech Z-5500s | Sennheiser HD-570 headphones | Samsung 2494LW |
Laptop - | Dell Inspiron 1420 | C2D T7300 (2.0GHz, 4MB cache) | 2GB DDR2-667 | 160GB 7200RPM HDD | DVD +/- RW | 14.1" WXGA+ (1440x900) | Broadcom 802.11g | NVIDIA 8400M GS 128MB |