120GB minimum.
60/64 is right on the edge of too small. You'll be spending more time than needed to manage the free space.
gytisxp :
32 gigs is more then enough for windows.
No, it's not. It will initially install. Then you start running the updates. I recently redid my daughters laptop with Win 7 Home Premium. After the updates, it was right at 40GB.
A small load of utilities (~1GB), and MS Office (~3GB) and it was 45GB.