Well, some CPU sockets actually have a little lever that you have to pull up, before you put the CPU in, and that would explain the CPU not being able to go in.
CPU: Intel Core 2 Duo E6300
Mobo: HP DX7300MT, Intel 965 Chipset.
RAM: 1GB DDR2 PC4300
GPU: Gainward GeForce 7600GT 256MB PCI-E
Drives: 1x160GB SATA2 HD, 1x80GB SATA2 HD, 1xLG DVD-RW