Microsoft have windows defender which help you protect in many way.

Windows Defender

Install this first.
Make sure you run auto update when your computer online generally.

This is necessary to prevent your computer to open harmful pages.
There is best thing out per my knowledge is spybot s&d

- Spybot SD

Why you need another uninstaller. because microsoft sucks.
You need this to clean up left over registry and files.

- Revo Uninstaller

It has been marked clean by download.com

You need this because “Remove Temporary files” is not good enough to do this.

- Ccleaner

It has been marked clean by download.com

For Antivirus you have two option.
It is basically slow on update as it is free, but good enough to protect you.
I am not certain what to suggest so choose on your own.

- Avast Home Edition

Go to link and download Home Edition, You need to register product though.
Don't worry it is free.

- AVG Free Edition

Go to link and down the free basic edition.

This is where you might tuneup your computer little bit.
Also, you can say thanx to microsoft.

- Live Scanner

Which clean up many stuff.

I found this on microsoft open source.

- SvcHost Viewer