Over the years, the following three causes for slow logons have been mistakenly identified as being relevant for improving logon speed on Windows clients.

1. Deleting your cached roaming profile.
   This will most likely do nothing or even make your logon slower if the profile is excessively big and needs to be pulled down from the server again (yes, that means you in cubicle 23 on floor 3b with the 6Gb of Mp3's in My Music)
   The only time this would help is if there was a problem with the local copy on the disk, but in that case you would probably be having bigger problems.
2. Cleaning the registry
   Registry cleaners in general don't do much for logon performance, although they might not be the same type of vapourware as Memory Defragmenters/Cleaners.
   The exception is if there is a known bug in a Microsoft or 3rd party component that is excessively bloating a section of the registry that the Registry Cleaner application is aware of (a couple of 3rd party printer drivers come to mind).
   On the other hand, something that has a snapshot of a specific point in time for the registry and could show you what settings have changed between then and now could be useful (but not related to slow logons).
3. Removing Temp files
   Temp files typically don't need to be flushed out, if you're at the stage where you're running out of diskspace because of Temp files then your System disk is basically undersized.
   The one time this would have helped was back in NT 4 days when there was a limitation in Explorer concerning the number of files that it would enumerate and this would cause hangs if browsing to the Temp folder.
   It might look unhygienic to have your Temp folder stuffed, but the Temp folder doesn't roam with the profile by default so it won't affect the logon proecss.

See also the following articles for up-to-date information on how to fix slow logon issues in Windows systems:

http://social.technet.microsoft.com/wiki/contents/articles/10130.root-causes-for-slow-boots-and-logons.aspx
http://social.technet.microsoft.com/wiki/contents/articles/10128.tools-for-troubleshooting-slow-boots-and-slow-logons.aspx
http://social.technet.microsoft.com/wiki/contents/articles/10123.troubleshooting-slow-operating-system-boot-times-and-slow-user-logons.aspx