turning .cdis back to .rars - help please

[SMBYKES]

can some one please explain to me how to turn a .cdi, or any big fie, into more than one .rar archive. some one told me he thinks you have to use dos and im ready to learn how to do it if some one knows how. i have alot of my games on cds and cant keep alot of files on my computer due to minimume space but once i get my 80 gig hd i would like to put all my games back on my computer for my server and i dont want to have kids download humungus files like those cdis. so if some one could help ide greatly appreciate it.