A 128GB SSD is good enough for Windows 8, but it depends on what other programs you want to install.
I believe a clean install of Windows 8 takes around 18GB. That will increase when updates are installed. Virtual memory (the Page file) is typically equal to the amount of RAM that you have; if you have 8GB of RAM installed, then the Page file will be 8GB. Generally speaking, people say that if the PC has 16GB of RAM, then there is no need to have a Page file in Windows assuming the programs loaded do not use more than 8GB of RAM. If you want the laptop to be able to hibernate, then Hibernate.sys file will likely be around 5GB - 6GB. Disabling hibernation will free up that space. Then there are Windows restore points which I believe defaults to using 10% of the C: drive capacity. Restore points can be useful, but it is something I disable.
Assuming your PC has 8GB of RAM a clean install of Windows could use up the following amount of space on a 128GB SSD:
18GB - Clean Windows install
8GB - Virtual Memory (Page file)
6GB - Hibernate file
0GB - Windows restore points. When 1st installed there should be no auto restore points. But over time it can grow to 10% of the C: drive capacity unless disabled or decreased.
That is a total of 32GB assuming 8GB of RAM. It can be decreased to 26GB if you disable hibernation. It will grow from there as you install Windows updates.