In the past, I have simply waited for a hard drive failure to replace a PC. A friend pointed out that perhaps this was not the most optimal strategy, since it involved downtime, panic and hair-pulling. What is a reasonable timetable for PC replacement? To give you an idea of my current machines, one is Vista and one is Fedora Core 11.
Clarification: This is for a small business.
It all depends on what your required MTTR is, and how much money you've got to throw around. In all honesty, though, in most cases PC hardware, properly maintained, is reliable enough over it's useful lifetime (the time until it becomes obsolete from a performance standpoint) that there's no point along the curve you can say "the risk of this component failing is now unacceptably risky".
If you need resilience from hardware failures, you either want hot spares and good backups. If you're only trying to guard against hard drive failures, RAID-1, properly implemented, can be a reasonable mitigation strategy (be warned, though: done badly, you'll have more failures because of RAID than it prevents).