Here is an idea of the software I'd install on my own computer in order to keep it running smoothly. Bear in mind that some applications require more experience than others to use (ie. sometimes it is difficult to know which scan results to remove and which to ignore with the registry cleaners; as such, it is best to be cautious when deleting entries).
avast! loads on start up and is always left running for the real-time protection. [Anti-Virus]
CCleaner is run about once a month. [Registry Cleaner]
Ad-Aware is run about once a month. [Adware/Spyware Cleaner]
Spybot Search & Destroy is run about once a month. [Adware/Spyware Cleaner]
Glary Registry Repair is run every few months or as needed. [Registry Cleaner]
Auslogics Registry Defrag is run once or twice a year. [Registry Defrag]
Windows Disk Defragmenter is run once or twice a year. [Drive Defrag]
Malwarebytes' Anti-Malware is run only as needed. [Adware/Spyware/Virus Cleaner]
In my testing most of these programs have been equally effective with XP and Vista.