I have been using Diskkeeper (Full version) and it always seems to do a good job.
I have downloaded AVG and one of their options is AVG tune up (trial), so I tried it, apart from the erors it found , it reported my main disk was 59% fragmented and proceded to defrag it, with a result of 0% fragmentation
I have now re-checked it with Diskkeeper and it shows a lot of fragmented files and unused disk space in the middle of the drive.
How do diffeent defragmenters get their fragmentation sums?
Sunnystains is right -- each defragmenter does things differently (and some, like Diskeeper, do things much better than others).
He's right again in recommending you keep Diskeeper as it will defrag your disks automatically and prevent fragmentation (also, it is VERY transparent while it defrags, meaning you don't have to stop using the PC while it's defragmenting.
AVG makes excellent anti-virus but I have seen many complaints about the AVG Tune-Up suite, as it tries to do too much and ends up doing nothing particularly well (as is true for most program "suites").
Check out the Defragmentation Information Center for more info on what Diskeeper does so you can use it to its full potential: