Batch file programming : A+1=b??

i am writing a piece of code in batch file to check if a drive exist or not here it is
-------------------
#-Link-Snipped-# off
if exist C:\windows (
echo ~%d0 Found>ComputerDrives.text
)
else (
echo Not found
)
Pause
---------------
now what i want is if i can manipulate the drive name in for statement i.e.
for(i=a,i=z,i++)
but the problem is the alphanumeric letters are not increasing with arithmetic operator ,and in batch file there is not an option of enum.
What can i do??