Boot it up and get an elevated command prompt (admin credentials). Type in SFC /SCANNOW and be patient, as this command can take a while to perform.
You must not stop it once it has started, so please do not do this as a quick fix. You are scanning for system files, and if it finds some stuff not in the correct place, it will fix them. Also, see if booting into safe mode speeds things up. If it does, then something in your startup isn't booting right. Try deleting the last thing you installed, or last two or three things if you have alot on the same date. Let us know if you need assistance with removing the programs!