Method 1: Use the Properties of Your Local Disk

1. Open My Personal.

2. Right-click the local disk volume that you want to defragment, and then click Properties.

3. On the Tools tab, click Defragment Now.

4. Click Defragment.

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Method 2: Use Computer Management MMC

1. Begin Computer Management MMC (Compmgmt.msc).

2. Click Disk Defragmenter.

3. Click the volume that you want to defragment, and then click Defragment.

Answer:
Install Diskeeper 2008 Pro and enable automatic defragmentation.

http://www.diskeeper.com

That's it! Diskeeper will automatically defrag the drive in the background, when required. You can download trialware at the above link (it's their official site)