Firstly it is pobably best to defrag your entire hard drive so go do something else for an hour or 2.

When the initial defrag is complete the fun stuff starts, now you defrag your .gcf files dont worry a full easy to follow guide is here it tells you exactly what to do.
www.youtube.com/watch?v=ccmoToUgEpo
Again defragging you ,gcf files may take a while also if you have many VALVE games installed but the end result should be worth it.

Or if you are lazy you can just download this zip file and extract to steammapps folder then click "defrag.bat"
files.resistance-teste...defrag.zip