You need:

- A good AntiVirus program, constantly scanning in the background
- At least two Spyware detection and Removal programs (AdAware, spybot)
- A decent 'Uninstaller', which monitors the programs being installed on your system and ensures that they are properly removed later.
- A good set of computing habits, including a reasonable working knowledge of Windows tools such as Disk Cleanup and Disk Defragmenter
- Sensible web browsing habits and an awareness of what type of "Can I install?" requests to keep clear of.

With those things, you will almost certainly never have a need to call in any 'Doctor' to fix up the problems that have been allowed to arise.

All of those things can be found for free!