Registry cleaners……..?

Are there any free registry cleaners? Will they improve processing speed? I have a Pentium III, 512mb RAM, and 80 gig hardrive with about 45% used. Processing is slow of late.

How do I increase windows xp boot-up time. I have cleaned the programs from the start-up list. only comodo firewall and nvidia driver components are there.
Cleaned my PC from anti-virus, spyware and other malware.
Cleaned my registry.
Defragmented the registry and the hard drives.
Disabled unneeded devices.
But still my boot-up time is 1 minute. From the moment of the POST message to the usable desktop.
PC specs:
Intel p4 ht 3.20GHz.
512MB RAM
Nvidia Ge force 6600.
D945GNT motherboard.
160GB hard drive space.
Already used Advanced windows care and disabled the floppy drive seeking.