I think dishing out the extra money for another HDD is a bit much. If you really feel your computer is running slow, take the following steps beforehand:
1. Registry Cleaner- I can't tell you how many people have way too many problems with their registries, get one and scan!
2. Anti-Virus- You should have this anyways, but run a scan periodically (Once every couple of weeks should be good)
3. Uninstall any unnecessary programs etc. from your computer that you no longer use, or you just dont need anymore
4. Go through your documents, pictures, videos etc. and delete old stuff
5. If you install a program, make sure to delete the installer when you finished with it, some programs such as Adobe programs have installers as big as a couple of Gigabytes
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Those are a couple basic steps you should take before considering a new Hard Drive.
IF none of these work, you can back up all your documents, and just perform a clean install of whatever OS you use. Then, if the problem persists, you should buy a new HDD, otherwise just follow those steps
Hope i helped!
-Bio