visual studio .net setup: I am trying to install Service Pack 1 on VS2005 Professional on a Windows
2003 Server (SP1) with NET Framework 2 with all critical patches installed.

When installing VS2005 SP1 (common version for Std/Pro version) this comes
up with the following error:

The installation source for this product is not available. Verify that the
source exists and that you can access it.

I originally installed VS2005 on a non-default folder in C:\program
files\vs2005 and I am wondering whether this is the issue?

I have had VS2005 working for about a year with no issues beforehand. I
have installed all VS2005 components and I also have the SQL2005 studio
components installed with SP1 and the hot-fix build 2153 - all following MS
guidelines.

I have the original VS2005 disk in the CD drive when I try to apply VS SP1.

try this

http://blogs.msdn.com/heaths/archive/2006/09/22/767053.aspx

using this fix sorted the problem and SP1 installed sucessfully.