Microsoft Security Essentials is free and so far works well on all my systems.
http://www.microsoft.com/security_essentials/
If your new box came with an OS disc, I would format and reinstall the OS without all the crap that the vendors install to supplement the cost of the computer. Only do this if you are comfortable loading OS' and updating drivers.
A great tip would be to get the new box running perfect with all your software, get another hard drive, make a clone and put the clone in a safe place. Back up all your important files like pics, music, spreadsheets, etc online. Then if anything ever happens, you can restore to brand new in a few easy steps.