Norton / Symantec has come a long way since the "old days"... Symantec have streamlined it and many reviews rate Norton 360 v3 as one of the best AV products for 2009.
http/www.pcauthority.com.au/Review/146185,norton-360-version-30-impressively-fast-and-comprehensive.aspx
There is a free 30 day trial if you want to trial it.
If you do trial it and are unsatisfied, remember after you uninstall, to use the symantec norton removal tool to finish the job:
http/service1.symantec.com/Support/tsgeninfo.nsf/docid/2005033108162039
My vote though would go to Avast AntiVirus.
1. Very good at keeping all the nasty stuff from my computer
I like the inclusion of a boot-scan that Avast has. It scans before the operating system is fully functional to prevent malicious files from running and preventing their removal. This can be accessed any-time from the GUI and has a run-once option on install.
Screensaver option, I also use this to scan my pc's when they're idle. It will resume the scan from where it left off if interrupted the next time the screensaver kicks in.
Auto-update feature is really non-intrusive. I have never noticed a lag in my pc while the update is running in the background. I only notice the update when the little blue box pops up in the bottom right hand corner telling me it is complete. No need to schedule or manually perform updates.
Resident protection is also very good. When browsing the net if a malicious web page is detected you are given the option of aborting connection or ignoring. With malicious file download, again you are given the choice whether to delete, quarantine or ignore.
2. Not really interrupting my computer experience (no lag, ...)
As stated above,
"Auto-update feature is really non-intrusive. I have never noticed a lag...".
I must let you know that some people that have done the switch from AVG to Avast have noticed a slow down in boot-time. But once the OS is up and running no further slowdown. I don't know whether this will affect your system, I guess the only way to be sure would be to test the AV program.
3. Pretty easy to handle (With AVG I had some experiences where it just blocked and deleted files I needed from my computer and I had to put the antivirus off in order to use it)
As stated above,
Resident protection is also very good. When browsing...
I have never had a problem with Avast deciding what should be done with any infected files. I almost always opt for the delete option, and can say I have never been hit by any serious malware.
I have two other malware scanners that are on call, don't run resident. I update them and run full system scans periodically. Having on call scanners mean that system resources are free until I decide to run them. They usually only ever find the odd tracking cookie, but I run them regardless for exrta security. I use Malwarebytes' Anti-Malware and SUPERAntiSpyware.
As for cost, Avast is free, so you can try it for size and see if you are happy with it.
There are many that say you get what you pay for, but in my experience I haven't paid for security applications since I have owned PC's and been very happy with the security apps I have installed.