Please do a complete CUSTOM install using the 64-bit DVD - it will work.
Here are the specs of your CPU:
http://ark.intel.com/products/41316/Intel-Core-i7-860-Processor-8M-Cache-2_80-GHz
It is 64-bit capable.
Start over - do a format of the OS hard disk. Make sure that this is the only disk on the computer. Then proceed with the install.
Let us know what happens. (Don't forget to backup to an external drive before you do anything). I am repeating this because backups are very important.