Windows Installer is an engine for performing transactional installations. When installing a product for the fist time, most often few or no files to be installed are already present on the machine. But when upgrading or patching a product, most often those files are replaced so copies must be kept if an error occurs and the installation needs to roll back. The following describes where and why Windows Installer may require so much disk space. Extracted files A bootstrap application may be required for a number of reasons. A bootstrap application may compressed setup files into the bootstrap application, or they might all be included in a self-extracting archive. Bootstrap applications may also download files from a web site either directly to a location on disk or indirectly to the Internet files cache. This location is typically under %TEMP%. If the extractor did not delete these files, you can delete these files after installation has completed. Note that some installers m...