Though I agree w/Evan's answer (starting clean is usually easier in the long run), here are a few things you could try:
- Clean up old and out-of-date libraries. Completely clean out your NVidia drivers and install the latest. Remove any Firefox installations and reinstall fresh.
- Check out what services are starting up when Ubuntu initializes. Clean out anything you don't need. Are you running both KDE and Gnome? When I was running both recently, I noticed KDE (which I wasn't using) had a number of extra services running (e.g. MySQL) even when I was only in Gnome. Removing KDE helped a bit to speed things up. Are you running Compiz? It can be a resource hog, and if your system can't handle it, I would recommend removing it.
You mentioned your system is sluggish. This can be a hard thing to quantify. Are certain applications taking longer, or have you noticed the system itself responding more slowly? If it's the latter, are you sure you have enough resources available to run the system? If it's the former, are your applications up-to-date? I know some of my favorite programs (e.g. Eclipse) have a track-record of poor performance. Upgrading them usually helps.