'computer optimisers'? Someones problably trying to sell you a crock full of brown goo. (or snake oil, if you prefer that term.)

Step one : Spybot S&D. Removes much nastiness in the form of spyware, malware, nasty cookies, nasty registry entries, an the like. This one's free.
Step two : decent antivirus. NOD32 is a good one, and it sure don't cost as much as Norton.
Step three : claer out the registry. You can partially do this by hand, but it's not really recommended (you'll only scrape the surface, as you can only recognise the keys with plain-english names. Most have names that look like MD5 hashes.) Registry Mechanic works good, though it's pay-for software.
Step four : Look at what you have installed. Look in your system tray, your Program Files, and ask yourself : 'Do I Actually Use That Thing?' Any program that fails this test gets nailed.
Step five : Profit!!!

Well seriously, that's all I have to do to keep my boxen airborne. Reformatting every so often (every year or so) helps, because desipte your best efforts bad stuff will creep in slowly, and a reformat (or some serious bit-twiddling) is the only to get rid of it, and often the only way to fix up a really bad computer is to reformat it (or spend 5x the times fixing manually ).