One of the common questions from our clients is "How long should I keep my server?". There's no set lifetime for a server, but we do have some guidelines that we follow.
When we sell a client a server, we strongly recommend purchasing a three year warranty with same-day parts delivery. Most manufacturers will also sell an extended warranty for years four and five. The reason they won't go any further than that has to do with the "bathtub curve". After a server has been running 24 hours a day for five years, it's very likely that the mechanical components such as hard drives, tape drives, and fans will begin failing.
Major updates to the operating system and other software running on the server generally comes out in a 3 to 5 year cycle. We had Windows 2000 Server, Windows Server 2003, and now Windows Server 2008. Even a computer at the top end of the performance scale when Server 2003 was shipped will not provide similar performance running Server 2008.
Some clients base the replacement decision on the depreciation guidelines. I have an arrangement with my tax accountant -- I won't offer tax advice and he won't offer computer advice.
There is a good likelihood that your business needs have changed over the years that your server has been in operation. You have either added or removed staff, increased the amount of tasks your server is used for, or upgraded other portions of your network. Getting a newer server that more closely fits your requirement makes sense.
Internet Solver will recommend to our clients that they plan on and begin budgeting for a new server between the third and fourth years of service for their server. This gives enough lead time to account for a new server in the budget and do the migration process in a controlled fashion instead of during a crisis when your server is having problems and needs to be replaced immediately.