Halp! QAQ

[SolluxCaptor]

Hi, everyone~! Ok, so I'm having some issues. I bought Amnesia from Steam a bit back (not that long ago). But I seem to be having some... lag/frame drop/ floor graphics issues.

I have a Dell Inspiron 1501. I also have ATO Radeon Xpress Series graphics card. Even when I put EVERYTHING on lowest setting, I get lag (at the main menu and during the game) but when I pause in the middle of the game, it doesn't lag at all.
When I'm walking, the floor seems to come up. It's very annoying. I just want to play the game in peace. If anyone can help me, I'd appreciate it a while lot!
-Nat