@ toca180,
your system is woefully underpowered. As Hektic pointed out - you need to upgrade. And if you're upgrading you might as well go for 1080P playback for the little extra it would cost.
I built my system some months ago: Gigabyte mobo, AMD 6000+ dual core, 4Gb memory, GTS8600 and an LG Blu-ray/HD drive. Plays 1080P flawlessly. Without the drive you should be able to upgrade for around £150-£180.
Take a look at
www.ebuyer.com