Go to www.ubuntu.com
and click Get Ubuntu. This will download a large .iso file. Once the download is complete, use your favorite burning software to burn this .iso file to CD as a "CD image". Then, pop the CD in your laptop, turn it on, and it should boot into Live CD mode, allowing you to try Ubuntu with no change to your computer. Try it for a while, and if you like it, there is an Install icon on the desktop.
More detailed guide here: https://help.ubuntu.com/community/Installation
To answer your second question, no anti-virus software is needed with Linux!
(edit) I just remembered that some Acer laptops don't have CD drives, if this is the case, your easiest option is to use an external USB CD drive. If that's not possible, you can also make a bootable USB flash drive that functions just like a Live CD: https://help.ubuntu.com/community/In...n/FromUSBStick