Extract the .rar into Computer > C: > Program files (x86 if running a 64-bit matchine) > Steam > Steamapps > > Portal > Portal > Materials > Models > portals. If the folder isn't present, create the folder and extract the files into there. Then right-click on Portal on your Steam games list > Properties > Set launch options... Then, on the line, type this line in:
To check if your PC can handle it, follow these steps:
If you're running XP, select Run... and then type in dxdiag
If you're running Vista or 7, type in dxdiag in the search bar
Then look downwards. If you're running DirectX 9 or higher, you should be able to run it. You might experience a little bit of lag when you make a portal near you or look into a portal.