scan

i am running window7 ultimate on one of my laptop.
i noticed in last few days that my computer ( internet etc.) dead slow, though i run disk clean up -windows defender every 24 hour. (def'rag regularly)
what it could be? i hope i have no virus present in the system !
do i have to install good virus scanner ware? if so what could be the best ?
any help will be appreciated.

Jacee said:

Install and scan with Microsoft Security Essentials (it's free)
http://www.microsoft.com/Security_essentials/

thanks Jacee
just downloaded and installing it.