Debian with XFCE would be my recommendation, a combination of a really stable distribution with a really stable desktop environment that is also very light on resources and relatively fast.
I also recommend Debian because, echoing what Brickwizard stated about the vast majority of penetration testing focused distributions being built upon a Debian base, and thus knowledge gleaned from using Debian should translate into being able to use and maintain such a distribution better.
Debian is sometimes considered to be a moderate expertise required distribution to install and use (in no small part due to non-free software and firmware requiring extra steps to install and source), just a fair warning, but with a sensibly old computer hardware support * should * be relatively good.
If you do not feel comfortable with Debian proper, Linux Mint Debian Edition would be my other recommendation.
