The next question is then if its a good idea to have and run multiple antivirus/antimalware/antispyware etc software at once.
never install multiple AV as this will give you a slow machine, windows errors and false detections

follow instructions and remove one http://www.avast.com/en-eu/faq.php?article=AVKB11#artTitle

if you want a second scanner, use Malwarebytes www.malwarebytes.org
if you use removable drives use MCShield usb protector www.mcshield.net