[RaZEr]

Можно:

var
SerialNum:Pdword;
a,b:Dword;
buffer:array [0..255] of char;
begin
new(SerialNum);
if getVolumeInformation('c:\',buffer,sizeof(buffer),SerialNum,a,b,nil,0) then
label1.Caption:=IntToStr(SerialNum^);
Dispose(SerialNum);
end;