Problems with Certain HL2 Props

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Hi there.. As of late i seem to have been getting a lot of problems with certain Half-Life 2 Props, such as these:
[IMG]http://dl.dropbox.com/u/1380197/rp_c18_v10194.jpg[/IMG]
[IMG]http://dl.dropbox.com/u/1380197/gm_flatgrass0075.jpg[/IMG]

I would be very grateful if someone can help me rectify this.

PC Specs :

AMD Athlon 64 x 2 Dual Core
3.5 GB DDR2 RAM
Nvidia GeForce 8400GS , 512MB.

It seems I have only been getting this lately, and only with certain Half-Life 2 props.