I'm not too familiar with bootcamp, but my understanding of it is that it's Dual Boot and not VM? In any case, the 5770 probably can't handle that resolution. I run the same video card off my desktop and I can play it somewhere between medium and high at 1680x1050, try lowering the resolution:
Here are my rendering settings for my 5770 (in User.ini)
[Rendering]
AllowAntialias=1
AllowBloom=1
AllowBlur=1
AllowCutsceneDOF=1
AllowDOF=1
AllowDecals=1.0
AllowMotionBlur=0
AllowRain=1
AllowSSAO=1
AllowScatterDOF=1
AllowShafts=1
AllowSharpen=1
AllowVignette=1
AtlasTextureDownscale=0
DanglesLimiter=0
DetailTextureDownscale=0
Fullscreen=1
MaxAtlasTextureSize=2048
MaxCubeShadowCount=1
MaxCubeShadowSize=256
MaxSpotShadowCount=3
MaxSpotShadowSize=512
MaxTextureSize=2048
MeshDistanceScale=1.0
ShadowQuality=3
ShadowedLights=3
TextureDownscale=0
TextureMemoryBudget=600
UberSampling=0
VSync=1
Some like to put UberSampling to 1 (as opposed to 0 or 2 set by the launcher) but I've never been able to tell the difference between 1 and 0 so I leave it off. Also, my video card has 1GB memory, so TextureMemoryBudget is set to 600. If you have less, or start getting crashes, you might wnat to lower this value.
Let me know how it goes.