The Windows file system might be inferior to the Unix, but it is simpler to understand, in my opinion.
Actually, that's just because you're more used to it
2. Anyway, on to your problem.
Start by looking at the disk layout; run the following and post the results.
(BTW: you should read up on each cmd we mention eg use search util at http://linux.die.net/man/
, or on your local system it would be 'man cmd-name'. man = manual pages )
# just in case, also do
fdisk -l #that's a lowercase 'L' BTW
3. for any dirs mentioned by the above cmds you, you can drill down looking for disk space with the 'du' cmd
4. Look to see if you have logrotate setup; should be under eg /etc/logrotate.d and main cfg /etc/logrotate.conf.
5. lets see what your system is doing
That lot should do for a start