Probably the .ISO file is corrupt.
How to fix:
You can try to re-download the .ISO file or use the VS SP1 Web Installer (which downloads only the required patches itself) and re-apply the patch, when asked to chose between re-apply and uninstall.
The .ISO file has the following checksums (according to Heath Stewart’s blog post):
- SHA1: 61C2088850185EDE8E18001D1EF3E6D12DAA5692
- CRC: C77C2A14
Updated (Sept 3rd, 2011) to include link to checksum of .ISO file. (Thanks to Ahmed Ilyas)