If you didn’t already know, ArcGIS 10.0 has added flexibility that previous versions didn’t have, including the ability to uninstall or roll back service packs or patches from within Add/Remove Programs. This is for the ArcGIS Desktop, Reader and ArcGIS Engine Runtime and may come in handy. For example, if you find a problem when testing with a service pack, simply uninstall to see if the issue remains.
Related Link:FAQ: Can an Esri Service Pack be uninstalled for Esri ArcGIS products version 10.x?