First question, what do you want to do?
There is mysql (
www.mysql.org), which is a full blown SQL server, command line only, but powerfull (IIRC ENWorld uses MySQL as the back end).
What Platform?
I tend to start off by looking at
www.gnu.org and see if the program I am interested in is available.
There is open office (
http://www.openoffice.org/) which has a spreadsheet and now, by the looks of it DB support. If is free, apart from the 60mb download and multi platform.
Minimum Windows specs are
# a Pentium-compatible PC
# Microsoft Windows 98, NT, 2000 or XP (2000, ME or XP required for Asian/CJK versions)
# 64 MB RAM minimum
# 250 MB hard disk space