Do you have a Time Machine back up from BEFORE you upgraded? It's no use making one now and then reinstalling Snow Leopard - it won't be able to read any TM backups you made in Lion/Mountain Lion.
There are quite a lot of threads on how to restore your old system, including this user tip here:
However, if you don't have a backup from your old 10.6.8 system, it's going to be a bit more painful.
Some things that you don't like about ML can be fixed (others, we have had to live with since Lion ).
You can reverse the track pad behaviour to normal in Trackpad prefs, you can turn scrollbars to always on, (System Prefes > General) and, if you need those PowerPC apps, you can always re-install Snow Leopard from your DVD on a separate partition or external disk and use it just for those.
If ML is slow, that's undoubtedly a function of conflicting software (Assuming you've got decent enough ram and hard disk space). Remove everything from your login items and restart the computer. Update all your third party apps and junk any old kexts. If you need help with this, just post back and let us know.
None of my ancient (power PC) applications work anymore and I've got to spend a fortune on new versions of Photoshop, Microsoft Office etc.
Note that you're going to need to update those applications sooner or later. Eventually you will need a new computer - though you could certainly delay that for a long time with Macs, usually. When you do buy a new computer, it won't be capable of running 10.6, and thus won't be capable of running those apps. Better to upgrade those apps now, so that if you drop your computer or something you don't have to pay for new hardware and new apps at the same time!