Dont buy from shops, every pc bought from a shop is -

1. A rip off
2. Made from cheap bits
3. Not as stable (imo)

Every computer i have built has always been stable as hell. The computer i am typing this on now has been running none stop for over 2 months now. I only switch it off when i have to to put new bits in. Another pc that me and my brother built is a webserver, which has an uptime of over 400 days until the ups blew up, (sods law). If you get decent components then you shouldnt have a problem.