The drive is probably dying!!! A drive which is becoming very slow is often a symptom of a dying drive.
Actually you're quite lucky you are still capable of accessing the drive, oftentimes the fails completely.
First thing to do: back up your drive.
Then switch your computer off, run to the store, and buy a new HDD.
Actually, it is quite possible that you have another problem. The drive may be fragmented. Virusses, ad-ware and spyware may cause your system to slow down. Windows itself may become slow (depends on the version).
You can download a test utility from the drive manufacturer website to test your drive. If it shows errors your drive may die any moment.