|Without more facts, I cannot tell if the diagnosis is correct, typically a slow system is not a bad hard drive unless there are other symptoms. Explain better and give us more details in order to help you better.|
To test hard drive: Download ISO image of DOS version of Seatools from Seagate and prepare a bootable CD. Then boot to it and run the longest test. This will confirm any drive problems and if they are minor, it will fix them, if it needs replacing, it will tell you. The test utility from your own drive's mfg. might be even better, but you would need to confirm the mfg. first, Seatools will work on nearly all drives.
For a slow system, Run Malwarebytes to confirm you do not have an infection that slipped past your antivirus program. Remove anything it finds. Then run a Disk Clean Up on your drive (in Computer, right click on drive, Select Properties and click on Disk Clean Up), select all from the list and run it. You should do this about monthly and about weekly you should clean out your browser's History, Cookies, Temp Internet Files, etc. as well as emptying your recycle bin. If you have too many programs running in the background, your system will be slow. Uninstall any programs you do not use at all. Run msconfig, go to the Start tab and uncheck all except Microsoft and your antivirus program, Apply and restart the machine (you can later add back anything you find that you need by just running it again and adding a single check as needed). Another thing that can slow down a computer is having less than 15% free space on the hard drive or partition.
You have to be a little bit crazy to keep you from going insane.