I would leave mine on, but it is in my room, and i want my sleep lol. I haven't noticed any slow down and i've been turning it off before i go to sleep and turning it back on when i get home for 3 months now. I haven't defragged it in awhile........ I've heard many things about it, either way, your computer will last about the same, whether you leave it on or not.
If you are worried about power consumption, set the computer to go into standby mode, and to turn off the hard disks after a few hours of non use. Also set the monitor to go on standby. you only really need that if you leave your comp on all the time.
AMD Athlon XP 2500+@ 200x11 - EPoX 8rda3I - Thermaltake 420watt silent PSU - 2x512MB PNY PC3200(DualChannel) - TT HSF - PNY 5900SE, 128MB DDR, 256-bit - - Toshiba SD-R5002: DVD-RW - NEC DVD-RW - 2 Antec 80mm fans - 1 Delta 92mm fan - 1 Antec 120mm fan- 1 TT 120mm fan - Windows XP Pro - WD special edition 80GB 7200 RPM- Seagate 80GB 7200 RPM 8MB cache - Onboard Sound - Onboard Ethernet
ACER Travel Mmate 4000LCi - Intel Pentium M 710 - 15\" XGA TFT - 512MB DDR333 - 40 GB HD - DVD-rom/CD-RW Drive - WLAN 802.11b/g - Winxp Home - Intel Extreme Graphics - Average Battery Life: 4-5 hours - 6.4 lbs