Have a look at the Inspiron 6400n(N1164015) with 1Gb ram £360 (£420 for 2Gb) vat and delivered to door, plus all free open source software you can manage held in the Ubuntu repositories. I have this running for father, uncle and other family members in simple home surfing using Firefox, Thunderbird with Enigmail and VLC for AV duties, it all just works. I know the software and operating system work but I have no knowledge of this Dell hardware but I expect it works as well as any other laptops from big named shops.

PS You should not have to worry about various virus infections and with the right Fiorefox and Thunderbird extensions installed their machine should be about as secure as it can be without unplugging from the internet.