My old PC has a 512M RAM and when I open too much tabs in Firefox browser window it becomes very slower.I know the reason is the low memory,but I didn’t have a mind until I read bobwdn and mike555‘s comments in ubuntuforums

Following steps come from bobwdn:

Step 1: backup your computer is strongly suggested,then if you have an issue you can start over.

Step 2: Remove unnecessary apps and packages via Synaptic (exclude apps that you would like to use.In my case,I didn’t uninstall compiz).

Open Synaptic Package Manager from System->Administration menu.Search following packages and select for “complete” removeal:

evolution, evolution-common, evolution-couchdb, evolution-exchange, evolution-indicator, evolution-plugin, evolution-webcal, indictor-one, empathy, empathy-common (synaptic will prompt you to remove nautilus-sendto-empathy, so tell it agree), mono (now, the mono selection is going to cause synaptic to pop-up many, many other programs that will be removed with it. I agreed to all and have not had a problem, YET!!! The list included: f-spot, gbrainy, libart2.0-cil, libflickrnet2.2-cil, libgconf2.0-cil, libglade2.0-cil, libglib2.0-cil, libmime2.4-cil, libgnome-keyring1.0-cil, libgnome-vfs2.0-cil, libgnome2.24-cil, libgnomepanel2.24-cil, libgtk2.0-cil, libluanchpad-integration1.0-cil, libmono-addins-gui0.2-cil, libmono-adins-gui0.2-cil, libmono-cairo2.0-cil, libmono-corlib2.0-cil, libmono-data-tds2.0-cil, libmono-i18n-west2.0-cil, libmono-posix2.0-cil, libmono-security2.0-cil, libmonosharpizp2.84-cil, libmono-sqlite2.0-cil, libmono-system-data2.0-cil, libmono-system-runtime2.0-cil, libmono-system-web2.0-cil, libmono-system2.0-cil, libmono2.0-cil, libndesk-dbus-glib1.0-cil, libndesk-dbus1.0-cil, libnunit2.4-cil, mono-gac, mono-runtime AND tomboy)

In addition,continue selecting for “complete” removal:

gwibber, gwibber-service, bluetooth (Synaptic prompts to remove bluez-utils, I agreed), computer-janitor (Synaptic prompts to remove computer-janitor-gtk, I agreed), espeak, (Synaptic prompts to remove gnome-orca, libspeak1, python-speechd AND speech-dispatcher, I agreed), espeak-data (Synaptic prompts to remove gnome-bluetooth AND gnome-user-share, I agreed), bluez (then below bluez was bluez-alsa, bluez-cups AND bluez-gstreamer, so I saw no reason to keep those and remove them also), and finally compiz (Synaptic prompts to remove compiz-fusion-plugins-main, compiz-gnome, compiz-pluginz, compizconfig-backend-gconf AND libcompizconfig0, and I agreed to remove those, also.)

Step 3: Reduce startup applications.
Go to System–> Preferences–> Startup Applications and un-check: “Evolution Alarm Notification (evolution is gone)”, “GNOME Login Sound”, “Remote Desktop AND User folders update”.

Step 4:Finally,right-click ‘envelope thingy’ and remove it from the panel which remove the volume control icon. Then add them back to panel,the volume applet’s back and not the envelope thingy.

Reboot and Done.
Check the thread on ubuntuforums