Windows performance can take a hit if the registry is in a bad shape. I have found that the following tools are quite helpful:

1) Regseeker: Lookup any keword in the registry and delete entries as appropriate. The tool also lists problem entries in the registry which can be then manually dealt with.
2) MS Sysinternals PageDefrag: Allows the user to defrag specific files (some related to the registry). Lists the count of fragments of these files and user can choose to defrag if she thinks fit.