you might want to get a freeware registry cleaner (like regseeker) to clean the registry IF you have sufficient ram and you are sure computer is free of viruses and/or other malware.
Registry problems may make computer run slower but this should be one of the last things to check for.
First, like they said.... RAM, SPYWARE, VIRUSES, UNNECESSARY STARTUP PROGRAMS, UNNECESSARY BACKGROUND PROGRAMS are things that should be checked first.