Your computer simply is too old. Just live with it.
Moving this discussion to the Installing Flash Player forum.
Mine installs fine just never works, always says blocked plug in.
I have installed, uninstalled, emptied disabled plugin folder of flash.
searched, re uninstalled, re installed, rinse, lather, repeat.
iMac (Early 2008)
Processor 2.4GHz Intel Core 2 Duo Memory 1GB (one 1GB SO-DIMM) of 800MHz DDR2 SDRAM; supports up to 4GB