I run a whole lot of systems 24x - some have been up for at least 10 years.
I prefer to use SCSI drives in mission critical systems.
Here is what I do when the systems are NEW:
1. Replace crappy bearing fans with high quality ball bearing fans (psu, case, processor, video, etc)
2. Install as many case fans as the case permits (most come with optional fan mountings but no fans).
3. Ensure good cooling for all components (CPU, chipsets, memory, HD, other drives, adapter cards). This usually involves some reversal of fans and/or one or two strategically placed interior baffles.
4. Neaten up all cabling with wire ties. Flat cables are folded or replaced with round cables in critical cooling situations.
5. Install and configure monitoring software so that I get an alert when any critical fan is BEGINNING to show signs of problems.
6. UPS on every computer to provide clean power.
7. I "lubricate" all power connectors with a conductive grease to prevent resistive connections due to corrosion or tarnishing -- and low voltage --> HEAT.
Here is the quarterly maintenance schedule:
1. Check all fans
2. Blow out all dust including the PSU
3. Check temperature logs
Here are the failures over the past decade:
1. Multiple Seagate ST34572W SCSI HD (bad design? - I don't use this model anymore after 6 failed drives. I have 2 that still work - any takers?)
2. 1 Seagate Cheetah 18.3Gb SCSI HD
3. Many IDE drives - mostly WD
4. 1 PSU
5. Multiple Floppy and LS120 Drives
6. 2 CD-ROM Drives
7. 2 Motherboards - Capacitor Problems - Swelled/Leaking/Popped
I find IDE drives last about 1-2 years (1 year warranty) or about 3-5 years (3 year warranty).
SCSI drives seem to be built to last forever unless I do something really stupid. I've had only isolated SCSI failures.
The worst enemy is HEAT. Then comes dust and "varnish" that builds up inside the case which leads to HEAT and failure.
I have dual Pentium 200's that have run for over a decade. Many other machines that are within a year or two of the decade mark.
Machines that are powered on/off daily seem to go south with all sorts of maladies within 6 months to 2 years no matter what the quality of the components. Memory, Motherboards, HD, FD, CD, Fans top the list.