+1 for Avira Free!
Download & Install the following 3 freewares. It forms a free complete security suite for your system!
1. Avira Anvivir - http://www.free-av.com/en/download/index.html
Avira is best & light solution for internet security, real time updates, excellent overall system protection.
2. Malwarebytes - http://www.malwarebytes.org/
Malwarebytes is one of the best & effective antimalware tool out there.
3. Ccleaner - http://www.piriform.com/ccleaner/download
CCleaner is a system optimization, privacy and cleaning tool with an effecient & most useful registry cleaner.