mov [mousedup],eax     ;1 y x sta(0 0 y<0 x<0 1 btnm btnr btnl)
    bt  eax, 24+4
    sbb eax,eax
    mov al, [mousedup+2]
    add ax, [mousex]
    cmovs ax,[zero16p2-2]
    cmp ax, 640
    cmova ax, [$-2]
    mov [mousex],ax
    bt  dword [mousedup], 24+5
    sbb eax,eax
    mov al, [mousedup+1]
    neg ax
    add ax, [mousey]
    cmovs ax,[zero16p2-2]
    cmp ax, 480
    cmova ax, [$-2]
    mov [mousey],ax
    mov [mousedup],dword 0x100
    zero16p2:    

shr eax, 8
bt eax, 16+4;instead of 24+4
sbb ebx, ebx
mov bl, ah
add bx,[mousex]
;.. etc...
    

it's not that messy. I think you should give some space between the two blocks (one block for x and one for y).

why do you write "eax" (the first instruction) if your only going to read it back in for y a moment later? personally I'd just use a second register and read from it instead.

Code:

shr eax, 8
bt eax, 16+4;instead of 24+4
sbb ebx, ebx
mov bl, ah
add bx,[mousex]
;.. etc...
    

I have my own mouse code but it provides a couple other features so it's not comparable.