If your going to be buying I suggest getting Kaspersky. I've used it for a long time. It works very well and if anything happens it has a recovery disk with an OS to boot to separately so that even if Windows can be booted you can still try to scan/recover
PS: I suggest when you get your pc taking an old formatted HDD or a external HDD and going through windows to create a system image and a recovery disk. Its generally the first choice of recovery when your pc crashes.
Anti Virus: http://anti-virus-software-review.toptenreviews.com/
System Image Windows 7: http://www.howtogeek.com/howto/4241/
System Image Windows 8: http://howto.cnet.com/8301-11310_39-57604101-285/how-to-create-a-system-image-in-windows-8/