Windows 7: My computer is totally messed up now please help

I have so many problems on my computer now and I am thinking about reinstalling windows but that would cause me to lose all my data so I going to check here first. So here is the list of all my problems on the computer:

1. I get an error message when starting windows
2. Everytime I start windows it tells me to restart windows explorer. It even tells me to do this for some games.
3. I keep getting viruses and can never get rid of them all.
4. And here is my latest problem that started today. My uninstal software menu says I only have 25 programs installed on my computer even though I have over 100. This is causing a lot of problems and having icon pictures not to show.

I probably have more problems but these are the ones I need fixed or else I will have to reinstall windows and lose all my data so please help.

This can cause all of the problems:
Backup your stuff (and be aware that it may have viruses). Don't use it until you can scan it with a working antivirus.

Wipe the hard drive with DBAN or KillDisk (both are free - google for them).

Then reinstall Windows and see how it works.