Windows with a full complement of security updates, Microsoft updates, additional programs like ms office, 7-zip, Adobe, flashplayer, Antivirus, and all the other stuff that has a working relationship with the OS can easily top 80Gb without any extraneous stuff like games etc. Most important to remember is with an SSD you must leave at least 10-20Gb unused space or the SSD will slow down drastically.
A 64Gb SSD isn't a good idea, unless it's part of a larger build and used for swap file, page file, temp file alone. Minimum 120/128Gb or you'll spend more time cleaning out the drive than you will using it.