Hi
There is space to install Windows from DVD of Win 8 or 8.1
but installing 8.0 then updating to 8.1 usually results in a backup of Windows 8 in Windows.old
after a year or so of updates the space is much bigger
You would have no problem with a 120GB SSD if you also have a hard disk for storing videos and games
My Win 7 (x32) partition has used 65GB and Win 8.1 (x64) about 50 GB
(Win 8.1 is smaller as it is more recent and has less Windows updates)
(both with office, acrobat, Norton AV etc and most data on a 1TB hard drive
regards
Mike Barnes