The name says it all I didn't have an Anti-Virus and windows suggested some to me. (I had Kaspersky I just needed to get the ISO from my friend). It asked me if I wanted to use the Kaspersky Windows 7 Beta, which I thought it would last 15-30 days at the most. It lasts for about half a year, which is better then using something like Free AVG. It works rather well, it might be a little confusing for beginners. It did suggest other anti-virus's for me but I chose Kaspersky, so the other anti-virus programs might have a beta program too. (although I can't find all of those) Enjoy Obviously for more information look Here
No, I've used Kaspersky for quite a while and noticed no slowdowns. My PC has 3GB of RAM and a 2.1 dual core processor. I think it does do the scans really slow and accurate. Since the scans are so slow it doesn't seem to effect the speed of the PC. It is extremely cautious about things. even when you you messenger for the first time with it, it will ask if messenger is okay. Also if you download things and it finds a trojan it will stop the download and automatically quarantine the trojan. It will also stop things if it finds a code intrusion.