How many gigabytes are on windows 7?

32-Bit: 16GB

64-Bit: 20GB

Keep in mind your operating system will grow at least 10GB within short time due to update's and software claiming storage in your operating system folders. Windows should always have at least 256MB. free space to operate normally.

Put 128GB to your operating system is a good idea if you are planning to install big apllications and games on another Hard Drive!