if it has 512mb ram, look at lxle and wattOS.
if 1gb ram, look at linux mint xfce and linux lite.
good tips:
--- install "preload".
--- change swappiness value to 10 or a bit less;
https://help.ubuntu.com/community/SwapFaq#What_is_swappiness_and_how_do_I_change_it.3F
--- use lighter programs, such as:
music - guayadeque or audacious
videos - gnome mplayer
similar to paint on Windows - pinta
word processing - abiword
spreadsheets - gnumeric
youtube without the web browser bloat - minitube
basic text editor - leafpad
terminal - lxterminal
task manager - lxtask
web browsing - midori (with iphone user agent selection option)
email - claws mail or sylpheed
software add/remove: synaptic package manager is better than using software center
picture manager - gthumb
disc burning - xfburn
picture viewer - ristretto or gpicview
psensor is generally a good temperature monitor
there are other alternatives.