Registry fix programs general cause more harm then good IMO. I never use them.
I'm not sure what you asking about using the old drive.
General what slows down an OS is Spyware/ Addware.
Most repair shops clean your system of this crap to improve your speed.
However if it is too far gone then it's cheaper to format and do a fesh install due to the time VS $$ factor.
If your going to do the fresh install your going to need the windows cd unless they have it installed on another partition on your drive. Seems most of the store units have some sort of a restore feature build into the boot up profile.
Keep in mind you will loose all your old data when doing a format/ install. But you will have a brand new OS to screw up again.
The other way is to try and clean it yourself.
A few good free programs to use are:
Note: When running these get all the updates for them first.
Also it's best to turn off system restore while cleaning your system.
And Spybot I would run in safemode first.
There are other programs you can use but these are the first 2 I turn too.
If they can't clean your system you will at least know what problem your dealing with then attack it from there for that specif problem.
May the force be with you on your quest for lost speed.