Uninstall any old applications that are not needed, check disk usage analyzer, switch to Xubuntu and uninstall Ubuntu, and clean out junk files like hte Firefox Cache. Also check Synaptic Package Manager for any applications that are not needed.
To get Xubuntu-desktop:
Type this in a terminal:
sudo apt-get install xubuntu-desktop
When it prompts you for a password, type it in. Don't worry if you can't see anything.
Press y or enter when it asks if you want to continue.
To run Xubuntu, just log out and under options, click sessions and click Xfce. Choose as your default.
Now, uninstall GNOME by typing:
sudo apt-get remove alacarte binfmt-support brltty brltty-x11 capplets-data cdrdao cli-common compiz compiz-core compiz-fusion-plugins-extra compiz-fusion-plugins-main compiz-gnome compiz-plugins compiz-wrapper compizconfig-backend-gconf contact-lookup-applet dcraw deskbar-applet ekiga eog evolution evolution-common evolution-data-server evolution-data-server-common evolution-exchange evolution-plugins evolution-webcal example-content f-spot fast-user-switch-applet firefox-3.0-gnome-support firefox-gnome-support gconf-editor gdm-guest-session gedit gedit-common gnome-about gnome-applets gnome-applets-data gnome-control-center gnome-desktop-data gnome-menus gnome-netstatus-applet gnome-nettool gnome-panel gnome-panel-data gnome-pilot gnome-pilot-conduits gnome-session gnome-settings-daemon gnome-spell gnome-terminal gnome-terminal-data gnome-themes gnome-user-guide gnome-utils gstreamer0.10-plugins-base-apps gstreamer0.10-pulseaudio gstreamer0.10-schroedinger gstreamer0.10-tools gvfs-bin gvfs-fuse human-icon-theme human-theme hwtest hwtest-gtk libao2 libart2.0-cil libasound2-plugins libcanberra-gnome libcanberra-gtk-module libcanberra-gtk0 libcanberra0 libcompizconfig0 libdecoration0 libdeskbar-tracker libdirectfb-1.0-0 libebackend1.2-0 libedata-book1.2-2 libedata-cal1.2-6 libedataserverui1.2-8 libeel2-2 libeel2-data libegroupwise1.2-13 libexchange-storage1.2-3 libexempi3 libflickrnet2.1.5-cil libgconf2.0-cil libgdata-google1.2-1 libgdata1.2-1 libgdiplus libgif4 libglade2.0-cil libglib2.0-cil libglitz-glx1 libglitz1 libgmime-2.0-2a libgmime2.2-cil libgnome-keyring1.0-cil libgnome-pilot2 libgnome-vfs2.0-cil libgnome-window-settings1 libgnome2.0-cil libgnomevfs2-bin libgpod-common libgpod3 libgtk2.0-cil libgtkhtml-editor-common libgtkhtml-editor0 libgtkhtml3.14-19 libhyphen0 libicu38 liblpint-bonobo0 libmono-addins-gui0.2-cil libmono-addins0.2-cil libmono-cairo1.0-cil libmono-cairo2.0-cil libmono-corlib1.0-cil libmono-corlib2.0-cil libmono-data-tds1.0-cil libmono-data-tds2.0-cil libmono-i18n1.0-cil libmono-i18n2.0-cil libmono-security1.0-cil libmono-security2.0-cil libmono-sharpzip0.84-cil libmono-sharpzip2.84-cil libmono-sqlite2.0-cil libmono-system-data1.0-cil libmono-system-data2.0-cil libmono-system-web1.0-cil libmono-system-web2.0-cil libmono-system1.0-cil libmono-system2.0-cil libmono0 libmono1.0-cil libmono2.0-cil libmtp8 libndesk-dbus-glib1.0-cil libndesk-dbus1.0-cil libneon27 libopal-2.2 libpisock9 libpisync1 libpt-1.10.10 libpt-1.10.10-plugins-alsa libpt-1.10.10-plugins-v4l libpt-1.10.10-plugins-v4l2 libpulsecore5 libsamplerate0 libschroedinger-1.0-0 libsdl1.2debian libsdl1.2debian-alsa libsgutils1 libspeexdsp1 libsqlite0 libtracker-gtk0 libts-0.0-0 libwps-0.1-1 libx11-xcb1 mesa-utils metacity mono-common mono-gac mono-jit mono-runtime mousetweaks mtools nautilus nautilus-cd-burner nautilus-data nautilus-sendto nautilus-share openoffice.org-base-core openoffice.org-calc openoffice.org-common openoffice.org-core openoffice.org-draw openoffice.org-emailmerge openoffice.org-gnome openoffice.org-gtk openoffice.org-impress openoffice.org-math openoffice.org-style-human openoffice.org-writer pkg-config pulseaudio pulseaudio-esound-compat pulseaudio-module-gconf pulseaudio-module-hal pulseaudio-module-x11 python-beagle python-gmenu python-gtksourceview2 python-uno rarian-compat rdesktop rhythmbox screen-resolution-extra sg3-utils sqlite sqlite3 syslinux tangerine-icon-theme tomboy tracker tracker-search-tool tracker-utils tsclient ubuntu-artwork ubuntu-desktop ubuntu-docs ubuntu-gdm-themes ubuntu-system-service ubuntu-wallpapers untex usb-creator usplash-theme-ubuntu vino whois wv xdg-user-dirs xdg-user-dirs-gtk xulrunner-1.9-gnome-support && sudo apt-get install xubuntu-desktop
in a terminal. Note that you should do this when running under Xfce. Xfce is also faster in addition to being clean and good looking.
DO NOT GET XP. IT USES MORE HARD DRIVE SPACE THAN LINUX.
Take a look at http://www.psychocats.net/ubuntu/index
Hope this helps.