Check out the knoppix cheatcodes http://www.knoppix.net/docs/index.php/CheatCodes. I also get the same error on some PCs. The code I would use at the boot up splash screen is: knoppix nodma
If that does not work, reference the above link and try others like noapic or failsafe.
As a side note, some people had problems with Dell Optiplex systems. If I recall correctly, they had to get a new bios from the dell support site and flash the machine to the lastest version. Search these forums for the phrase dell optiplex to find out more about this.
I hope this helps.
James