driver sweeper is no longer recommended as development has stopped and will actually cause issues with the newer nvidia drivers
while registry cleaners can help in rare situations they often cause more problems than they fix as they rely on a database of what the makers of the cleaner feel should be in the registry and any thing different is seen as a error and removed. (eg run cccleaner on a system with the latest audigy 2 ZS drivers for windows xp and it will break the EAX console, requiring the drivers to be uninstalled and manually cleaned then reinstalled.
Most memory booster apps actually slow the system down because they don't actually unload background processes, they simply cause a chance of events similar to what happens when you launch a large program that is requesting a large amount of memory. Items not related to the active process are pushed to virtual memory. This causes issues such as when you try to do anything beside use the 1 program you freed up memory for, the system remains laggy for a while as it moves things from virtual memory, to memory again.
windows 98 benefited from a registry cleaning before launching a large application but windows xp and up does not need one as the OS does a good job of freeing up memory when a program request a large amount of memory (memory for idle apps are moved to virtual memory as the program request more memory.
don't believe me, launch a large program while it is launching you will see memory usage go down a little then shoot up, and as the program uses more you will see this happen more and more, the memory usage goes down slightly before going up. a memory booster does not do this as this process requires the OS to actually monitor the program you want to run