Download & Install the following 3 freewares & Keep them always updated. 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 excellent overall system protection & internet security with real time updates, .
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 efficient & most useful registry cleaner.
The Best Security & System Optimization combo for all types of systems.
If You don't want Avira Means, Install Microsoft Security Essentials.
Both Avira & MSE are good & use negligible system resources only!
I prefer Avira!