To get a reliable system I will generally specify good quality components i.e. Asus or Gigabyte MBs with solid capacitor power designs, with a good quality high efficiency power supply such as OCZ, coolermaster etc in a Coolermaster ATCS case. This allows me to pull out the MB for inspection and cleaning directly from the case without disassembly. To keep fans clean for reliable cooling.

The processor cooler is always over rated for the heat dissipation you can expect from the processor, even if it means a very large expensive ($50) copper heat pipe cooler with a 12cm fan i.e such as the Zalman CNPS9900. I will generally always de-clock the processor in the summer and run the processor at normal speed during the winter so that the processor is generally not any warmer than I am, i.e 37 C.

If building a RAID array internally you will need a case which can cool the drives adequately or specify the slower 5400 drives such as the newer 5400 speed ecogreen drives from Samsung or Greenpower WD drives into a RAID setup. The Coolermaster case has fans directly in the front on the hard drives. They only consume a few watts each when idle and abour 5-7 watts at full tilt. They run very cool and are cost effective and have good MTBFs and performance is not radically effected. I would use a Solid State Drive as a boot drive i.e. no moving parts for your OS disk. These SSD drives have MTBFs as good as any enterprise Scuzzy Disk and consume virtually no power again keeping the system nice and cool.

http://www.wdc.com/en/products/Products.asp?DriveID=336

Keeping the computer cool, quiet and efficient does wonders for reliability and there should be no reason why a well specified PC shouldn't run smoothly for 5+ years.

The Althon XP 2800/Asus A7N8X deluxe with Windows XP I'm using at the moment is getting on for 7 years old with its 120Gb WD Boot drive and 200GB RAID drive setup.

BTW I don't ever intend to buy any other Microsoft OS ever again, simply because Microsoft made me pay for Windows XP to fix the problems caused by Windows 98SE. I reckon Microsoft should have paid me the same amount it cost me to fix Windows 98SE for the time and expense of using Windows 98SE along with a written apology. I can't believe they actually have the cheek to ask folks to pay for Windows 7, which is essentially a stripped down Windows Vista in an attempt to make the Vista code stable and reliable. It should be given away free with a letter of apology for all the poor Vista users along with a postal order of $200 value.