proc DlgProc uses ebx esi edi,hwndDlg,uMsg,wParam,lParam

local dummy dd ?

mov eax,[uMsg]
mov ebx,[wParam]
cmp eax,WM_INITDIALOG
je InitlizeApp
cmp eax,WM_CLOSE
je EndApp
cmp eax,WM_LBUTTONDOWN
je DragApp
cmp eax,WM_RBUTTONUP
je EndApp
cmp eax,WM_PAINT
je Paint
cmp eax,WM_TIMER
je Timer
cmp eax,WM_COMMAND
je BtnClk
cmp eax,WM_ERASEBKGND
je ProcessedMsg
xor eax,eax
jmp Finish

InitlizeApp:
push [hwndDlg]
pop [hwnd]
invoke SetTimer,[hwnd],1,1,0
invoke SetTimer,[hwnd],2,100,0
;stdcall uFMOD_PlaySong,Song,0,XM_RESOURCE
stdcall InitlizeGFX
jmp ProcessedMsg

DragApp:
stdcall DragObj,[hwnd]
jmp ProcessedMsg

Timer:
.if ebx = 1
stdcall DrawGFX
.elseif ebx = 2
stdcall TrnOptions
.endif
jmp ProcessedMsg

Paint:
stdcall CpyGFX
jmp ProcessedMsg

EndApp:
stdcall GFXShutDown
;stdcall uFMOD_PlaySong,0,0,0
invoke EndDialog,[hwnd],0

BtnClk:
cmp bx,1
jne ProcessedMsg

ProcessedMsg:
mov eax,1

Finish:
ret

endp

proc CpyGFX

local ps PAINTSTRUCT

mov ebx,[hwnd]
lea esi,[ps]
invoke BeginPaint,ebx,esi
invoke BitBlt,[hdc],0,0,ScreenWidth,ScreenHeight,[CanvasDc],0,0,SRCCOPY
invoke EndPaint,ebx,esi
ret

endp

proc DrawGFX

mov edi,[CanvasBuf]
mov ecx,ScreenWidth * ScreenHeight
xor eax,eax
repe stosd
call DrawStars
stdcall SetText,180,87,PrText,[bmpFont],0
stdcall DrawBitmapImg,0,0,[MainImgData],0,0
invoke GetClientRect,[hwnd],rc       
invoke RedrawWindow,[hwnd],rc,0,RDW_INVALIDATE
ret

endp    

WS_CLIPCHILDREN and WS_CLIPSIBLINGS is your friend. Also engage WM_ERASEBKGND and return TRUE from it. Set hbrBackground brush to 0 in the WNDCLASSEX window structure (if using a non-dialog), and of course do double buffering if necessary in WM_PAINT. Spend as little time as possible doing calculations in WM_PAINT, do all calculations outside of WM_PAINT to save time in the WM_PAINT handler. Things you can do, create a dib section that is as large as the entire screen so you dont have to re-create it every time the window resizes. You save time that way.