PCI Graphics Card HP dc5100 MT

I am having problems installing a new PCI graphics card into my
desktop.

My PC is a HP dc5100 Micro Tower.

Graphics card is a Saphire Radeon 9250 128MB DDR 64-Bit PCI card.

I've previously installed a PCI soundcard in the machine no problem
but have had nothing but agro installing the graphics card.

I cannot get the machine to recognise the presence of the new card
either automatically or via manual detect new hardware.

The HP dc5100 has on board graphics built into the mother board.
Someone has suggested that this is the problem but I am unable to
disable this.

Can anyone help?