Fail. Return code: 4

After a problem with my computer, a Dell Dimensio 9200, I got a error on one of my 2 harddisks.

I did run Harddrive Diagnostics and this was the result:

Drive 0 WDC WD2500JS-75NCB3          Fail. Return code: 4

Drive 1 WDC WD2500JS-75NCB3Pass

My computer is very slow since that problem. I can reach my data and already made a backup from all my data.

Does anybody know what the problem is?

And how I can solve this?