Can't upgrade OS from 10.6.3. Keep getting an error. The Installation failed. An unknown error occurred.

I am trying to upgrade my desktop from Snow Leopard to Mavericks. I have upgraded to Snow Leopard 10.6.3 with the disk I purchased from Apple. I have the subsequent upgrade combos but they will not install. I have created a USB iso for 10.6.4 as well as 10.6.8 and I continue to get the same error message. "There were errors with the installation. You may want to try installing again. The Installation failed. An unknown installation error ocurred. The installer encountered an error that caused the installation to fail. Contact the software manufacturer for assistance."

Specs on the machine are: Mac OS X 10.6.3, Processor: 2 x 2.8 GHz Quad- Core Intel Xeon, Memory 12 GB 667 Mhz DDR2 FB-DIMM.

Model Identifier MacPro 3.1

The only thing I haven't tried is getting on my wireless at home to try to do the updates that way mostly because of the connection speed and I have a limited data plan.

Thoughts, suggestions, solutions welcome.