The initial install of SQL Server 2005 will always put about 570 MB on the system drive.
Most of this goes into the Windows global assembly cache (GAC) which can only exist on the system drive. There is a further approx 430 MB that can be put on a different drive, meaning that the initial SQL 2005 install needs about 1GB of disk space, plus room for your databases.
A SQL 2005 service pack will add a further approx 800MB to the system drive, mainly due to GAC updates. Some CUs have been known to add a further 500 MB to the system drive. Items in the GAC very rarely get overwritten or deleted, so the GAC tends to grow quite large over time.
To minimise disk space for SQL 2005 components, go directly to SP4 and allow for space to accommodate your CU strategy.
The disk space requirements for other versions of SQL Server differ, with SQL 2008 needing about 3,000 MB on the system drive for the initial installation and 900 MB for a SP.
Original author: SQL Server FineBuild 1-click install and best practice configuration of SQL Server 2017 2016, 2014, 2012, 2008 R2, 2008 and 2005. 14 Mar 2017
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