After Installing OS don't pay any money for any antivirus programs!
Download & Install the follwoing 3 freewares. It forms a free complete security suite for your system!
1. Avira Anvivir - http://www.free-av.com/en/download/index.html
2. Malwarebytes - http://www.malwarebytes.org/
3. Ccleaner - http://www.piriform.com/ccleaner/download
The Best Security combo for all types of systems.
Exactly the tools I would suggest.
Avira has very good internet security, real time updates, great overall protection.
Malwarebytes is one of the best malware scanners.
Ccleaner is a great all purpose tool. The most useful tool is probably the registry cleaner.