I have an dvd .iso and I want to put it on a cd but it is to big, is there anyway I can split it up then put it back to gether ?

Any program that can explore .iso images will let you copy files out of it. So just open the thing with winrar and copy the files to your hard drive. Then master them into separate iso images as you wish. The result won't be bootable of course.

If you want to put the iso on CDs for transport then you can use any compression program to make it into a multivolume archive.

This may sound like a stupid question, but how exactly do i make a multivolume archive using winrar or winzip???

WinRAR: In the "Archive name and parameters window" (the thing you get when you create a new archive) you see a "Split to volumes, bytes" box with a nice selection of predefined volume sizes.