What kind of resolution do you run it in? Even though we got good gfx card, even by today's standard, it can't handle everything
Try with the new RAM. If that doesn't solve it, my next guess would be the gfx card itself. Unless it runs fine on any other game that requires a good card to run. Like the game Oblivion for example. Or S.T.A.L.K.E.R.
You could try the old switch the ram around trick. Are you using 3x1024 sticks, or 2x1024 + 2x512? Some times you gotta be careful with what you put in which memory bank. Some times some ram doesn't like to be placed with another brand/type and so on ram. Some times if you use 2x1024 you need to use bank 1+3 or 2+4 and not 1+2 or 3+4. And some times the other way around. It's confusing, I know.
As for spyware/viruses, try
http://www.kaspersky.com/virusscanner free virus scanner, open in Internet Explorer. Kaspersky is the best antivirus program there is at the moment. And the scan is free, and relatively fast from web too.
Then there's always the clean-install option.... Some times Windows, any version, gets borked and needs a clean install. That's about it for now, I'll follow your thread and try to help if I can come up with more ideas.
Oh, and btw, I've heard about others having problems with the demo. Could be the demo is not optimized at tall. It's a demo after all.