I'd be prepared to bet that there's absolutely nothing wrong with your hard drive. They tend to work or not work. There's no middle ground where they simply slow down.
You might gain a minor advantage by defragmenting the drive but, given that Windows 7 is designed to defrag as it goes along, it probably won't help much. (To defragment and/or check the drive for errors, go to Windows Explorer, right-click on the icon for your C drive, select 'Properties' and then click on the 'Tools' tab).
'Slow running' is usually related to software, rather than hardware. Try the following:
1. Run a full virus scan with your security software.
2. Run a malware scan (using the free version of Malwarebytes:
https://www.malwarebytes.com/mwb-download/ )
3. Uninstall your security software (such as Microsoft Security Essentials, Norton, etc). Try using your computer offline to see if runs faster and then briefly try accessing the internet. (Stick to safe sites, such as the BBC, and you'll be fine!). Security software (and, in particular, the updating elements of it) can be a major cause of slow-running computers; if you find that your computer runs faster without t, try something else. (I recommend the free version of Avast:
https://www.avast.com//en-gb/index )
4. Download, install and run the free version of CCleaner:
https://www.piriform.com/ccleaner/download
(I'd normally advise against using a registry cleaner but CCleaner is safe and frequently recommended in the technical press)
5. Leave the Task Manager running while using your computer and keep checking for anything which looks suspicious. (Access the Task Manager via right-clicking on the taskbar at the foot of your screen. Click the 'Processes' tab and then on 'Show processes from all users' ). With the exception of 'System Idle Process', nothing in the CPU column show ever be in double figures (with the possible exception of your web browser occasionally). There shouldn't be many entries in the 'Memory 'Private Working Set' column over about 10,000. Any unusually large figures, in either column, might indicate something hogging your computer's resources. (Also look for a figure in the CPU column which keeps changing every second or two, even if the actual numbers are quite low. That can indicate a problem, especially if it's for svchost.exe, which is associated with something trying to download to your computer).