Yes as InfectionZero said try and boot into safe mode, if this seems like its back to the normal speed then it may be some program or service causing the slow boot. I think this is unlikely however its worth a try.
Its either the hard drive itself or its something that has gone wrong within the software.
When you have booted in is it taking ages to for example load up your documents or is accessing this like normal. you could check the processes on the task manager to see what may be using the recourses.
If it comes to it then you may need to do a recovery of the system files, this means you wont loose all your personal data but instead it will replace those files that the system needs to function. you could also try a system restore to a date that the computer was working. And maybe a virus scan.