There can be dozens of causes for slow boot-up but one common reasonis too many programs (many of them un-necessary) all trying to start up at once. There is a great free program called CCLEANER. It is a registry cleaner (which may also help) but it also has a utility that lets you choose which programs run at start-up. Worth a whirl, it's free after all.
Once computer has fully booted click on the start button in the search box type in "msconfig" without comers.
From the result click on the msconfig and a setup will open on the top menu select startup and disable everything less your security..then close and reboot..
One thing that will sort out sluggish system is a solid state drive..
I have win 8 desktop with ssd installed and full boot is seconds.
have you maxed your ram out, if you have any spare memory slots, buy more memory, lots you can do really. virus, malware scan, defrag ya pc alo like what has been said turn off any unwanted programs that u dont need running once in msconfig, have a look HERE