Hi nimmz,
This is no consensus on which is the ideal antivirus software - there's a thread devoted to that specific topic over
here 
However - and this is only my personal opinion - I am very happy with Norton Internet Security 2011 (Norton 2012 will probably be released in another few weeks). It is very light on the computer resources (the Norton programs were re-written from scratch starting with the 2009 products) and Norton offers very good support, especially if you go to the free user-to-user
Norton Community forums if you have a question.
Whichever antivirus software you decide to install in the end, always make sure that your computer fulfils the
minimum system requirements - you can find them over
here for Norton Internet Security (under the "System Requirements" tab).
Again, this is just my own opinion - there will definitely be more opinions and suggestions coming along in this thread soon

It's aways important to do a
lot of reading and research; asking for opinions is already a good idea (although getting different opinions can be confusing sometimes

); reading reviews and information is also very helpful. You can also a trial version of the antivirus software which interests you - just remember to by all means remove it with the vendor's removal tool if you don't want to stick with it and try out another antivirus program (do
not try out too many as this can slow down the computer sometimes).
Since you upgraded to Windows 7 and did not buy a new computer (which usually comes loaded with preinstalled software), it looks like you won't have to remove any preinstalled antivirus software. If there is any preinstalled antivirus software, it is strongly recommended that you use the vendor's removal tool (available on the vendor's website) to uninstall it cleanly before installing new antivirus software.
As an extra layer of protection, I also suggest that you install an on-demand (=non-realtime) protection program like
Malwarebytes free version, which is very popular with users. Just remember that you have to update it manually, since it has no realtime/auto-update function.
And finally, caution/wariness is also an important layer of protection.