It's the memory usage Fyron, not the CPU usage. If you have a program that you don't use tying up memory, then that will slow down the rest of the programs trying to use extra memory. A good case in point would be your explorer.exe program. When you boot up it runs at about 15 to 17k memory. As you continue on the file will build up to 30k. You can "end process" and then start it again with no problems, as long as you have nothing running on your computer. Once you do that it will drop down in size and start to build up again.
As far as defragging goes, I've read posts where people just lose files/programs on disks as over time, XP will write over files and programs. That's why they (HDD OEM's) suggest that you defrag every once in a while to allow the OS to rewrite/update the FAT. It's not about speed or optimization.
I just rebuilt my neighbors computer. He didn't have 'dust bunnies' he had 'dust elephants" in the unit. It is a great idea to use filters. Even just a thin piece of foam that will let air flow thru. My computer has an intake and exhaust fan on it. The intake face has to be wiped down about once a week or it just looks like s#%t, with all the dust clinging to the cover.