Antivirus software run smoothly, almost transparently, on dual and quad core processors. Intel Atom processors for netbooks are dimensioned for basic browsing experience, even the newest generation of Intel Atom processors (Atom N450 and N470) didn't bring a significant performance boost.
The game will definetly change with dual core Atom (N550, coming up summer 2010). In any case most of the Windows based netbooks sold in 2010 will have a single core Atom processor, for this reason you should be careful in choosing an antivirus software, avoid antivirus that produce a heavy CPU load... you don't want to slow down an already slow system!
The German magazin C't benchmarked 7 free antivirus suites
- Avast Free Antivirus
- AVG Anti-Virus Free Edition
- Avira AntiVir Personal
- Comodo Antivirus Free
- Microsoft Security Essentials
- Panda Cloud Antivirus
- Rising Antivirus 2010
Of the three "fast free antivirus" Avast provide the best protection since a Web Filter is included.
A major problem with free antivirus software is the "slow" update rate: once a day for the free antivirus vs a couple of hours for most of the commercial antivirus programs.
It is up to you to decide whether or not to invest 40-50 USD / year in a commercial security suite or to just use free antivirus. In any case you shouldn't miss a Web Filter function to avoid infections from malicious websites.
See also: Free anti malware/anti spyware protection for netbooks