Scanreg/opt is Scanreg run under pure DOS (not a DOS window) with an optimize/compact registry command attached.  That's why you would need to run it from an autoexec file, before Windows starts, or from any pure DOS file.

It's made by MS so they may have more documentation on it.  I have 3 or 4 registry cleaners and compactors and all of them have their pros and cons.  I had forgotten about this one - after all, who uses DOS anymore?

Con - I just ran it and it took about 10 seconds so that's time that's going to be added to every boot.  But I reboot several times a day because I boot into several different configurations - if you only boot once a day it's hardly going to be noticeable.

Pro - I noticed an immediate and substantial system speedup.  I wish I had remembered to check the size of my registry before and after.  It was so substantial that I'm considering adding it to my autoexec file.

As for the RAM cleaning my understanding is that some .dll's remain cached - inactive but still in memory.  Having frequently used .dll's in cache can speed up execution because they don't have to be reloaded while infrequently used ones slow the system by taking up RAM that could be used for other purposes.

I have an ancient system (2 yrs old, 256 Mb RAM) and I have my system.ini configured for conservative swap file usage (forces the system to use available RAM before using the swap file) so I use MaxMem by AnalogX (free at http://www.analogx.com/contents/download/system/maxmem.htm) which cleans out the RAM either manually or at a predefined level.  It has a nifty little systray icon that shows graphic memory use and it cleans up RAM left allocated by programs that don't properly free up RAM when closed (a lot of MS progs in this category).