Western Digital are the only company that still offers a 3 year warranty on their hard drives (to my knowledge) which means they are confident in their product.
I would reformat the drive instead of defrag it. Reformatting takes longer and you have to back all your shit up but it does a much better job since it starts your harddrive fresh again. Defrag can sometimes make matters worse.
Agree with Formant. Dont connect your DAW to the net. Theres too many viruses out there and running antivirus protection in the background will slow down your audio programs significantly. They should always be disabled when doing audio work. Not to mention that modems can fuck with IRQs making it harder for your DAW to perform audio related tasks.
YOU MUST HAVE A SEPERATE DRIVE FOR YOUR OPERATING & AUDIO. Never record or store audio files (that bneed to tbe streamed) on the system drive. When you have two hard drives you can partition them. Not just any old way. The outer of the disk spins faster than the inner. And Windows partitions from outer to inner. So Make a 20GB partition on the system drive and put your OS and other audio programs on that partition. The remaining space on that drive can be for storage (where fast access is not as neccessary). Then on the second drive, make a 30 - 40 GB partition to store audio files, samples, reason / protools (or whatever you use) sessions, virtual instrument sound files etc etc. Again, the remaining space can be used for storage.
This way, the OS and audio is retrieved from the outer of the disks (fastest) and the stroage is on the inner of the disks (doesnt need to be as fast).
But if your shit is slowing down, id recommend backing up ALL your shit, reformatting the audio drive and then wiping the system drive and reinstalling a fresh copy of the OS. Go back to how it was in the begining. If its still too slow, THEN look at upgrading your CPU.