Slow boot-ups can be related to how you 'close' the previous session.
Turning a Windows system "OFF" (from 'Start> Turn Off Computer...' button) is not the best way to put your system away for the night (or extended periods).
A vastly improved method is 'Hibernation'.
Shutdowns using the 'hibernate' method gives several benefits: It's a near zero power state; all 'Startup' folder items are already loaded, Windows background services are already 'up', and all open work moves to the hard drive, yielding really fast start-ups.
And if power is lost (storms, etc.) the document or picture you were working on won't be lost like it would be in 'Standby' mode, and the system will resume just where you left it.
For XP (newer OS's are similar):
Go to Start> Control Panel> Power Options> Hibernate: here check the box to enable.
To show this option when turning unit off; Start> Turn Off Computer> HOLD down 'Shift' key & 'Standby' becomes "Hibernate': tick that.
(Some systems have a 'crescent moon' key to go right to hibernate, or set laptops for this operation 'when lid is closed').
To start up, push the power button, and Windows will resume right where you left it, bada bing.
"Turn off" can be done once a week to keep things in order, or as directed from within an application install or update.
Use the Power button to shut down a PC as a last resort ONLY. Continued use of that will insure the OS will fail at some point.