I have just reformatted my 'C' drive on my Sony Vaio using Windows XP. Prior to this I had all sorts of 'checkers' working to protect it. These included CCleaner, PC Pitstop, Speed up my PC AVG 8 and Avira. All they seem to do is slow down the laptop and do nothing. They claim to have found all sorts of problems but never seem to speed up anything, only adjust the registry.

This time I downloaded Avira after installing XP and it found 4 virus's after an hours work. I hadn't even reinstalled any files or downloaded any other material.

My point is, do I really need to install any other checkers, Doctors or what ever, or leave well alone?

An interesting point, at one time I had 3 different registry cleaners on my pc, 1 paid for and 2 freebies from cover discs, if I ran one after the other and cleaned at the end of each run, then all 3 would find quite a few errors that the others missed and it didn't matter which order you ran them in either.

You're referring to some different classes of "checkers", there. Avira is an antivirus. (you almost certainly should have one of these.) Ccleaner is a file cleaner with a good rep, that includes a registry cleaner. The usefulness of reg cleaners is debatable. Some knowledgeable folk strongly advise against it, others approve, cautiously. Ccleaners' reg cleaner is generally regarded as safe. Some are not, and can do damage.
SpywareDoctor and Spybot are a different class again, that of antispyware applications. Should you actually get any spyware (or other malware such as rogue programs, trojans, etc) you will need one of these. (MBAM or Superantispyware are currently amongst the highest regarded.)

What you need is up to you. A lot of these programs are designed (or overlap into) security, which most think is important. Especially if you do online banking or CC transactions. Very robust security involving a two way firewall and a good AV at a minimum is recommended. One or two demand scanners for spyware are also considered wise.
As for cleaners, registry cleaners, and the various "tweakers" around, these are designed to make computer maintenance easier. What folk use tends to depend on what works well, and how well they want their systems to perform. The truly nerdish among us get into it...it's a continually ongoing process of making the PC run at its best. And like anything, the sky is the limit when it comes to security, tweaks etc. Those mentioned above are "entry-level" applications, which are very popular.

A good basic maintenance routine that would probably suit the average user would include weekly scans (or monthly if non-critical) for malware. This would follow a disk cleanup, defragment, and maybe a registry cleanup. At the same time all apps on the computer would be checked for updates, especially Java and Flash. These are often updated as a result of vulnerabilities discovered. I find a useful time to perform this maintenance is around patch Tuesday, when MS release updates (2nd Tuesday of each month.)
Sorry to ramble.
My computer fair romps along, BTW.:)