It depends on the version of Windows 7 that you have. As a good rule of thumb a minimum of 30GB is recommended for Windows 7 *(possibly more for Windows Ultimate).
If you want to run Windows 7 on your PC, here's what it requires:
1 gigahertz (GHz) or faster 32-bit (x86) or 64-bit (x64) processor
1 gigabyte (GB) RAM (32-bit) or 2 GB RAM (64-bit)
16 GB available hard disk space (32-bit) or 20 GB (64-bit)
DirectX 9 graphics device with WDDM 1.0 or higher driver
Some Additioinal information with regard to the "system reserved" Partition in Windows 7 setup:
If you do not want the 'System Reserved' partition to be created and existed, the best way is to stop Windows 7 installation process to create the partition when installing Windows 7.
In Windows 7, the feature (100 MB partition to store WinRE files) is installed on all computers if the OS is installed on hard disk with single partition scheme, or unallocated space (space which not yet been partitioned) on the hard disk drive.
Thus in order to skip or avoid the 100M partition to be automatically created during installation, here?s a few rules to follow when choosing where to install Windows 7 to:
1. Do not install Windows 7 to a hard disk that not yet been partitioned or to unallocated space (When install Windows 7 to unallocated space, no warning pop-up or confirmation is asked, and setup will straight away and directly create partition 200 MB of disk space as special partition without notification).
2. If possible, try to create all the necessary partition(s) and format the partition(s) before attempting to install Windows 7.
3. If you?re installing Windows 7 into a new hard disk, or a blank hard disk with no partition defined yet, or if you must delete all existing partitions to start afresh, chose Drive options (advanced). Delete (if applicable) unwanted partitions. Then, click New to create the single partition or multiple partitions according to your own preference.
When prompted with dialog box saying ?To ensure that all Windows features work correctly, Windows might create additional partitions for system files?, click on Cancel button. Optionally, to be double confirm, Format the partition before selecting it to install Windows 7
Finally, if you want to do some research; there are also some great articles, instructional videos and such to help with your Windows 7, installation, migration and upgrade decisions located at our Springboard site:
Thanks again and good luck!
Microsoft Windows Client Support