You're not old enough to have owned an Intel discreet video card. Consider yourself lucky for that.
If that's 99% of the games you own, you really should branch out into other titles.
Older games have issues with all sorts of software, it's not AMD exclusive.
Now then, are these legal copies of Portal and Portal 2? I've played both on my system with no problem at all, but cracked versions of any game can be prone to crashing. As I've advised many others over the years, if you have issues that you believe originate with the video solution, the best course of action is a complete uninstall of the entire driver suite. A registry cleaning followed by a clean install of the newest drivers can work wonders. Also take into account the fact that it is equally, if not even more so, likely that your problem could be with Steam itself. Have you ever tried using a different AV or firewall? Have you had problems with any other Steam games? It's also worth noting that AMD's drivers aren't the only ones installed that could be causing conflict. What are the rest of your specs?
No open source support is not the fault of the company. They have made the SDK's available. Can't comment on the proprietary drivers. Haven't used a Linux distro since I went back to school.
Yes, they are legal, unless Steam is a massive bootlegging source
I have no other steam games. The problems have been persistent over several reinstalls.
As for branching out, I play Star Wars Battlefront 2, and I hope to get KOtoR for christmas.
Besides, that was just a sample. I play Angband, Privateer: ASCII Sector and TOME. I know, all those roguelikes are kinda geeky.