Yes not installing drivers could be an issue, by guides do you mean Nick N's?  You may have to reformat and start from scratch.  However, if you wish to avoid that then add the highmemfix.

In Win7 the fsx.cfg is found here:
C:\Users\USERNAME\AppData\Roaming\Microsoft\FSX

Add the HIGHMEMFIX=1 under [GRAPHICS]

There are numerous other settings like [BUFFERPOOLS] that you  can add, check your PM's

Robert