I just wanted to say that you may not need a new hard drive.
I had this problem recently also. I was trying to extract a RAR archive and it was going in surges, and taking over 30 minutes when it should take 5 minutes. I also noticed through process explorer that the hardware interrupts process was eating a lot of the CPU.
About a month ago I had a bad SATA cable that caused my hard drive to stop working properly. I have since replaced the cable but this is actually what caused my current problem. Let me explain...
Basically hard drives are allowed to access memory directly, called DMA, which allows them to run really fast without bugging the CPU which should be busy doing more important things than being a traffic cop. However if they start getting errors, such as when you have a bad cable, the computer says 'woah, we better let the cpu take control' and then flips the hard drive over to PIO mode which is much slower. The kicker is that it never flips back, even after correcting the problem.
So to fix the problem, I went to Start -> Settings -> Control Panel -> System -> Hardware Tab ->Device manager. In here you will see an item called 'IDE ATA/ATAPI controllers'. Click the plus sign beside this and you should see one or many Primary and Secondary IDE controllers. Right click on each of them and hit properties, then click the advanced settings tab. This window has a drop down, this should be set to 'DMA if available'. Below this there is a box for 'Current transfer mode'. If this is showing PIO mode, you will have to hit cancel on the current window, then right click on whichever IDE controller that you were in, and uninstall it. Then reboot your computer. After this you should notice that it reverted back to DMA mode, and it should solve your problem.
Hope this helps someone. I registered just to post this.