Dinesh, does your processor have VT-x extension? If so, just tick the VT-x/AMD-V Virtualization Acceleration option in VirtualBox. FYI, you need to have VT-x/AMD-V to be able to virtualize 64bit guest OSes. I have OpenSUSE 11.2 64bit running as a guest right now, using VirtualBox 3.1.8 r61349.
Edit: I just read Intel's Doc about your T5500 here
, it doesn't support VT-x, thus you won't be able to virtualize 64bit guest OS. You can virtualize 32bit guest OS though...