With the price of HDD these days (at least where I am) maybe try what I did.
I had upgraded(through an insurance claim---legit

)my motherboard,CPU,RAM,HDD,Video card etc.
The new HDD was 240Gb and obviously running Windows and any other programs I installed---FSX and others.
Now as I understand it,when you load up a drive you give your computer more to read to achieve a given result and because everything you've got is on one drive it starts to get a bit cranky.
So what I did was use my old HDD 60Gb drive as an internal hard drive addition(it came up as F: in
My Computer ),
I transferred everything that was unnecessary for the running of the programs on C to F.
60Gb is pretty small and I ended up with that filled as well so it was off to the shop for another 240Gb drive to replace the 60Gb!!
Now I have 2X240Gb internal drives.
C is obviously the program runner and F is simply storage.
The 60Gb now resides in another older unit as F drive freeing up the rather pathetic original 40Gb original drive(the original package at sale when installed used 25 of that 40,no wonder it was slow!)
What we have then is your C drive wholly left to run the programs and not have to read anything else to do it. And the less you have percentage-wise on your main drive the faster it goes.
And 'Defrag' is the word of the day----do it!!!!!
An external drive might be your best option I just went with what I was comfortable with and I don't know about externals but I had to format the new drive on mine and also formatted the 60 to really clean it.
Hope that helps
Col.
White on White fly all night.
Red on White you're alright.
Red on Red you'll soon be dead.