If you want to use Ubuntu on an older PC, then memory might be tight. I recently got a HP/Compaq 7100dc which only has 512 MB memory, and will be used for light surfing plus web based e-mail. It does not have an attached printer.
The following command removes a number of services which are superfluous in the above situation:
$ sudo apt-get remove bluez brltty pcmciautils speech-dispatcher \
apport cups system-config-printer-gnome evolutionExplanation: this removes support for braille input devices, BlueTooth, laptop extension cards (PCMCIA), text-to-speech, crash reporting (to Ubuntu), printing and the memory hungry e-mail/calendar client Evolution.
If you are knowledgable about security, you can make the decision to remove AppArmor. More information here: AppArmor in Ubuntu.
$ sudo apt-get remove apparmor
Also, on such a machine it is wise to turn off all visual effects by going to menu System, Preferences, Appearance. Switch to the tab Visual Effects and select None, then click Close. Explanation: this switches your window manager from Compiz to the much lighter Metacity.
The above mentioned procedure saved me 30 MB, from 125 MB used memory to 95. To find more memory-hungry processes, take the following procedure. First, find a process:
$ ps -e -o rss,vsz,cmd --sort=rss | sort -n -r | head
Then find the path to the process:
$ whereis <processname>
If you have a path, find the corresponding package:
$ dpkg-query -S /path/to/programname
Then find out if you really need this package:
$ dpkg -s <packagename>
If you don't need it, you can remove it:
$ sudo apt-get remove <packagename>