A regular defrag is always worth doing. The frequency depends on just how you use your PC.

Before defragging claer out as much rubbish as you can & particularly Windows temp file & Internet Explorer temp files. Do this by My Computer, right click on the drive & select properties. Then on the General tab click on Disk Cleanup. Once it's completed it's analysis it'll tick the obvious ones for clean up . Just OK it.

You can do the same in Internet Explorer by Tools & Internet Options. There you'll see the option against Browsing History.

Once you done this try a defrag.