I was taking a look at Activity Monitor after seeing my performance was pretty terrible. 

I was wondering, does anyone know what "mdworker" does?

I have been watching it for a while and it is consistently over 40% when the CPU is not doing other stuff. 

Time Machine backups have not been enabled on this system and Spotlight indexing has already been completed.

There is one for me, one for "nobody" and one for "root". The only one taking resources is mine.

Any ideas would be greatly appreciated.

It is the new and improved Meta Data importer. Don't worry, after some time you'll see its CPU usage drop. It kicks in when there's any kind of file activity, especially documents.

Or you can disable it altogether if you don't use Spotlight. Just google "disable spotlight os x tiger".

I find spotlight useless, and have it disabled myself (along with Dashboard).

You guys are great, I typed "mdworker" in on google thinking of getting doctors or something. Oh and by the way, spotlight is better than anything on windows, and saved my butt a couple of times with losing a file. Also how could you live without Dashboard. That is my life. Oh and the file part complete confirms it, I just finished a huge power point, plus 2 90 paged documents, filled with pics

mdworker is a tool that indexes what, where, and what is in files.

its only noticeable and runs for more than a few seconds when you first install Leopard and if you attach a new drive with a lot of files for the first time. one your stuff is indexed you'll likely never notice it run for more than a few seconds.

hmm it kicked in here too, and worked for about 5 minutes... no new drives or documents added... but today I did try "iDefrag"... looks like it kinda messes up with indexed files... (I know defrag is almost totally useless, don't even start I just needed some contiguous free space for a new partition...)

Spotlight is an overkill for searching for files and content. It has some new features but tries to index at startup, when a drive is connected and so on... Really frustrating. I've disabled it completely.

Terminal, locate, find, and grep together are very powerful for searching stuff.