While CNET speaks rather positively of Diskmax in their
review, I would advise caution when dealing with any such software. In practice that amounts to making a
Restore Point prior to making changes to your system.
Some "cleaning" software makes a restore point itself and also offers a roll-back option. I did not find whether Diskmax has such an option (of course, I looked at the creator's website for five minutes only). If it does, then go ahead and use it (I imagine you will need to login to your administrator account when you do that). If not, look for a Restore Point created before you used Diskmax, as Greg suggested.
A also agree with Greg's opinion on "cleaners" in general and the CCleaner in particular. What I would like to add is that Windows provides its own disk cleaning tool. While this tool (right-click on any drive in Windows Explorer -> Properties -> Disk Cleanup) is not as comprehensive as CCleaner, it is certainly safe (does not modify the registry at all) and for most people effective enough. In my opinion your system becomes slower only if your hard drive is 99% full with less than 1GB of free space. If this is caused by strangely enormous size of the tmp folder or the Recycle Bin, the Disk Cleanup will solve the problem. Other than that, the actual performance gain after a clean-up is questionable.
At the same time, it's nice to keep your machine tidy. Here's what I do to this end:
(1) I set my browsers to empty their cache on exit (in these days of fast internet connections there is no reason to keep cached versions of websites; ignore this if you are using dial-up and like to see your web sites appearing instantly);
(2) I also set them to automatically delete almost all cookies (except for a very few useful ones, such as your New York Times or Google settings);
(3) I uninstall unnecessary software and manually delete any folders remaining after the uninstaller finishes;
(4) I
limit the size of the Recycle Bin - this make me to be careful when deleting large files though;
(5) I use Disk Cleanup every once in a while in order to empty the temporary folders.
Now, what happens if I run CCleaner after about a year of using the above? Sure thing, it finds lots of stuff to delete, especially if you use the registry scan. However:
Have I noticed any performance increase after I actually perform the actions suggested by CCleaner? No. All I get is the satisfaction of extra cleaning, i.e. deleteing another bunch of junk files.
Do I simply agree to delete all registry setting found by CCleaner?
NO! I do not delete any registry items unless I am sure that I know what those items are, what their purpose is, and how did they get there in the first place. Windows 7 is a very stable and well-designed OS. A few extra unused registry keys won't hurt, but deleting valid keys (or false positives from the view-point of the cleaning software) may in fact damage your system and in some (admittedly rare but still very unfortunate) cases lead to re-install and data loss.
and 
