Ok so i got fallout 1 i installed it played it and loved it. i got off my computer. the next day i log on to play it and i get an error mesage saying "Fallout 1 requires 20 megabytes of free space" i have 30 free disk space so help?

Do you mean 30 Megabyte or 30 Gigabyte?

gigabytes

Try running the game in Windows 95 compatibility mode in that case.

Yeah.
Fallout was built in the age of 1 to 3 gig drives; the routine that checks for free space is not prepared for the answer. IIRC Baldur's Gate as well, will warn you that you haven't enough free space to install it, when you have a really large drive... but that installer does allow you to try installing it anyway, and installs without a hitch.

Tell me about it, I've got the baldur's gate CDs still, even my old Blood Omen: Legacy of Kain CD amd it often says I don't have enough space for the biggest install.