also old XP machines might not be supported
fedora 18 NEEDS 2 gig of ram ( 1 gig ram bare min. to install )
but at least 2 gig of ram to run well and 3 to 4 gig ram is better.
also if you have legacy ATI or nvidia 3d card installed , they are not supported
please read the install guide
http://docs.fedoraproject.org/en-US/...ide/index.html