As posted in Best free antivirus for netbook, you can get a good antivirus protection for free and that won't slow down your netbook.
What I forgot to mention is that an antivirus alone is not enough: it is important to get anti malware and anti spyware protection. Something that none of the free antivirus provide.
One of the free anti malware/spyware (possibly the best) is PC Tools Threatfire. If you use a free antivirus I warmly recommend you PC Tools Threatfire in order to get a complete protection.
You can get it for free from the PC Tools, no need of registration.
I use it since weeks in my Dell Mini 10v (Intel Atom N270, 1GB RAM) to complement the free Avast Antivirus and I didn't notice a netbook slow down since the installation.