hard drives have a life span of maybe 3-5 years. sometimes less, depending on use, and environmental conditions.
Motherboard/processor/ram will last 10+ years.
A new hard drive is relativity cheap like 50-75+ dollars depending on speed, and size.
The question you need to ask yourself, is your computer fast enough for what you do. If all you do is email, internet, and some word processing. then a 2 year computer is worth getting a new hard drive for.
If you want to play new games, run heavy computing programs (3d content creating, cad, scientific stuff, databases, etc). then you might want to get a new computer.
Since a new computer will do those tasks much quicker. And time is money when the computer is your business tool.
Don't listen to those nay sayers. I'm not a big fan of dell, but they make an ok machine. They use generic parts, most computer companies use generic parts. I've seen dells that are 7+ years old run perfect with brand new hard drives.