Hard drive, hard drive hard drive.
This is the bottle neck in the performance of every computer. The CPU does trillions of calculations and memory does GB/s and the hard drive comes limping along, not even close to catching up.
The normal old school drives have 3 platters that spin at either 5400 rpms or 7200 rpms. There is 1 arm that moves, reads and writes the data to the spinning platters.
When you start windows you have services, drivers, programs and all that loading up. The drive can only do 1 thing at a time, so everything must wait its turn, thus the drive queue. The more programs and such the slower the boot up of course, also as a drive gets older or is having trouble it slows down, making the system slower as well.
This is why SSD is nice right now. It opens that bottle neck far better. No moving parts, drives are up to 20X or more faster. I put a SSd drive in a old 9 year old xp machine and it went from a 7 min boot up to a 30 sec boot up (The old drive was 54oo rpm and it was dieing, painfully)
So while disabling things and making less need for the drive will always make things faster, remember that if nothing has changed and it is getting slower then that could be a sign something is up with the drive. But no matter what, if you are able, get an SSD. Ever since I went to them I cant stand running off my old standard drives lol
Shane