I've been leaving my computers on 24/7 for 8 or 9 years, and I've never had any problems. I always make sure my monitor goes to sleep when I'm not using my desktop to reduce power draw, but I've never had a problem doing so. Even my laptop, which I've had for the last year, has been on 24/7 with the exception of trips across the country (just flight time) and it has held out remarkably well considering both it and my desktop have been running the Distributed.net cruncher so both have been at full CPU load the entire time. I didn't think my laptop would hold out so well being on at full load for a year, but it's done so admirably.