Probably will take a fair while if u havent done it before. After youve done it once it should speed up considerably as first time round the HD will be very fragmented. As the name suggests it defragments the HD, therefore sorting the files on the HD in order to make it run more efficiently/quickly. Also worth doing a "Disk Cleanup" if you surf the net alot as this gets rid of alot of temporary files that arent really needed increasing free space on ur HD and again making it more efficient. Hopefully thats about the size of it. I stand to be corrected.