I just got a newsletter from Raxco, the maker's of PerfectDisk which is a commercial defragging tool. They are having a sale, through 30 June 2008, of a 25% discount on any PerfectDisk product purchased online by using coupon code DEFRAGME25
We have had lots of discussions here on defrag tools and, in my opinion, the best ones cost money. Raxco PerfectDisk and Diskeeper are the 2 biggest names and seem to have the best products. My personal preference is PerfectDisk, but I have nothing against Diskeeper. Both products have a trial version and are Vista compatible.
The regular price for PerfectDisk 2008 Pro is $40 (25,80 €), so the 25% discount makes it $30 (19,35 €). 2008 Pro is what I would recommend for the home user like me. If you need more features or an industrial solution, they make other flavors for that. By comparison, Diskeeper 2008 Pro is $50 with no discount. I don't know if that version of Diskeeper is equivalent to PerfectDisk Pro -- again, there are different flavors.