You dont use md5 summer on the iso itself, but on the md5 sum file.
What speed did you burn the cds at? You must burn isos at a slow speed, they are notoriously bitchy about going at 52 speed burns.
Go to nero 6 ultra, int the Nero StartSmart, turn on the 'expert' button or whatever that BS is, its at the bottom right I think. Its the button with the two people on it which look like the MSN logo.
Go over to the "Copy and Backup" part, it is the second to last icon to the right. Select the "Burn Image to Disc". It will pop up a menu where you find the iso on your computer and hit ok. Remember to finalize the disc (im not sure if this matters but it might. Also do "disc at once").
Then, when it burns and takes a while, find the md5 sum on this disc and use the md5 summer for windows on it. Or whatever, there is acommand line one if you ahve linux on your system. I dunno what it is.
Repeat for all the others. If you do not want to burn it just to check the md5 sum, with nero 6 ultra there is another option. Go to the "Extras" part, its the icon all the way to the right in the Nero StartSmart, and click the "Mount a Disc Image". It will pop up a box, select "Yes". It may give an warning from windows that it is not a signed thing, ignore it, it doesnt matter.
A box pops up and click the "enable drive" box. It takes a while, be patient, When it does, a tab will appear. Click the tab and click on the "..." and find the iso and open it.
Then go to My Computer and there will be an extra drive in the list, this is the same thing as if you would have burned the cd to a disc, except it is virtual. Then use the md5 summer to go in and check it.
Well, If none of that stuff works, I suggest fedora core 1. But if you do not mind a bit of trouble customizing your OS, go for slack
Slackware is a very versitaile disto.