should not slow down and an SSD likes to be half populated
short answer is no,
not with only 120g SSD, and do you have an HDD for storage to go with that? Load all games and apps on HDD and if you don't have one, GET ONE
and how are you only using 19g? Windows 7 is more than that and so is Windows 8. Are you on XP32bit?
but if you do want a partition, do it on a fresh install when you load Windows you have an option of making partitions then. Using a third party partition maker might lead to problems for you. Numerous threads on third party SSD partitions having problems.