For a slow a system there's a few things you can try out.
1)good free system registry cleaner
2)unload unnecessary startups in the msconfig utility's startup group.
3)analyze and defrag your OS drive if found with a degree of fragmentation.
RegCleaner and CCleaner seem to have earned some good names for freeware programs. Plus you need some security? Some links here have some good tools for both cleaning and security.
RegCleaner's home page,
Regcleaner
CCleaner's home page,
CCleaner.com
For security and antivirus/antispyware tools,
Grisoft's AVG 7,5 free antivirus,
AVG Free Advisor: Installation files & documentation
Other antivirus tools including AVG are found at
http://www.majorgeeks.com/downloads29.html
Lavasoft's Ad-Aware SE Personal and a new one called Spyware Terminator are getting some good reviews.
The free version of Ad-Aware is found at
Ad-Aware SE Personal Edition - Free Software Downloads and Software Reviews - Download.com
Spyware Terminator's home page is seen at
Spyware Terminator - Spyware & Adware Real-Time Protection ST also has a mild firewall built into it.