CompTIA
The Computing Technology Industry
Association, develops "Vendor-Neutral" Certification Exams. Its Mission is to advance IT industry growth through standards, professional competence, education and business solutions, with their developed exams to establish basic set skills of the topic.
Ubuntu Linux Cutting edge, user friendly, stable, large user community, wide hardware support, allows you to run it LIVE CD without affecting your current partition, allows you to install it in a "folder within Windows" and dual boot into it, or to dedicate a partition and dual boot, or run it all by itself on a Intel or Mactel. Also comes in flavors for, students, lower end/older machines, students, and "Media" edition. Can't go wrong, as it is also FREE!
Vector Linux
Excellent for lower end/older machines. Uses slimmer & leaner GUI interfaces for faster speeds. Includes:
Fluxbox
JWM
MWM
TWM
Xfce4
Has many "legacy" drivers to install your hardware. There is a free version offered for download, in addition to a paid SOHO (Small Office, Home Office) version which you can download or order the physical CD's.