I'm trying to use the Highres mod on Fallout. But, every time I launch it I get the can't run 640 x 480 error.
I can see that there's a post on the forum that shows the fix for this, but my computer won't let me access it for some strange reason. Can someone please post it here for me?
Thanks.

You mean this .pdf?
http://etistudios.com/temp/resolution_error_fix.pdf
If you cant access that, then try installing a .pdf viewer. The common one is adobe acrobat. I find that foxit is many times faster.
In short, it tells you to go into the Nvidia control panel, manage custom resolutions, and create a 640*480 setting, refresh of 60 and bpp of 32. Im not sure how its mean to help since the drivers should be able to sort this out without user intervention, but it may be worth a go.