Official document says

来源: 于 2006-10-13 09:07:06 [旧帖] [给我悄悄话] 本文已被阅读：次

http://support.microsoft.com/kb/837783

Around 1500MB.

If you install Eastern Asia fonts and a little extra, 2GB or more is expected due to tons of security patches and its uninstall contents. That's the space required for average users.