I tried to run the game and here's the message I got. Funny thing it worked the last time I checked a few days ago. Of course I have well over 20 MBs of free space. So, what the hell is going on?

I've gotten this a few times too. I think it's because the check for disk space free probably overflows the variable (since the concept of having hundreds of GB of free space on a disk drive a decade ago was crazy). I've found that if I start deleting some junk and clearing the recycle bin, I can get it to work after a few tries. It's something that hopefully should be fixed, but until such a point, that's a workaround.
Edit: Found another post where someone has a better workaround:
[url=]http://www.gog.com/en/forum/Fallout_Series/not_enough_free_hard_disk_space/perm=4/#p_b_4[/url]

Meanwhile I noticed that thread too, sorry for making this one then. By the way it got "fixed", meaning that it worked after some tries - but it seems very random apparently.

It's not so much random as just really annoying, but it makes sense. Let's pretend that computers didn't use binary for now, 'cause it's easier to explain. :)
When Fallout checks for free space, let's say the variable they used would accept a value up to 99999 MB. If you have exactly 100000 MB free, then, that would essentially be treated as a zero, since it can't read the 1 (Or as -99998, if it's a signed variable type, which I think might be the case here if the configuration fix isn't always working).

Try starting up Fallout right after boot-up, before you do anything else. Then minimize it while you go about your other business. Works for me.

I had the same issue when I first installed the game. I didn't launch it immediately after installing, so when I tried to do it several reboots later it gave me the "hard disk space" error. I went to edit the CFG file, but since it had never run the file wasn't there.
I ended up uninstalling, verifying there were no left over registry keys and then reinstalling. I launched immediately after the install and it worked. Then I quit the game and attempted to start it again. "Hard drive space" error reared its ugly head. This time the CFG file did exist. After editing it down to zero I've had no problems playing the game since.

so i just went and rightclicked on the exe went to compatibility and i chceked run this program in compatibility mode for and select older windows, i checked win 95 for example