If you are running W7 then just download and install Microsoft Security Essentials in whatever bit your OS is "32bit or 64bit". As for as malware just click on run and run "MRT" which is the malicious removal tool you download each month through MS updates. This is good enough for most home users.
If you need a more robust paid anti virus then Eset, Web Root, Bit Defender or Kaspersky will do.
Their are other free anti virus programs but I find their advertising a little to intrusive and annoying to cope with.