You may be encountering a problem that sometimes occurs on older machines with small ram installations. Although the release notes say Mint needs only 512 Meg to install, it occasionally takes more. Not a problem on newer systems as most have 1 Gig or more, but on systems like yours it can be trouble.
One way around this without installing more ram is to partition the hard drive before you begin the installation using the partition editor from the live CD desktop or, if you can get it, a stand alone copy of Gparted (available free here: http://sourceforge.net/project/showfiles.php?group_id=115843&package_id=271779
Get the .iso file so it can boot ). If you do this and allocate at least 512 Meg to swap space, the installer can use the swap space as virtual ram to ease the installation.