If you don't have a policy about when PC will be replaced, they'll never be replaced.
[By "PC" I mean the laptop and desktops that people use, not the servers.]
In the server room there is usually more thought about when each device gets replaced. Your PC environment generally needs some kind of repeatable, cyclic, process so that it stays fresh. Without it things get old and unsupportable, or people get upgrades as a status symbol and it becomes political. With a good policy things get better and more cost effective.
A certain fraction of your fleet should be old; that's just economical. However, extremely old machines are more expensive to maintain than to replace. It is a waste of your time to produce a work-around so that new software works on underpowered machines. It is a waste of your users' time to wait for a slow computer. It is bad time management and bad for productivity to have seriously old machines.
Companies often get into this situation. Sometimes they "save money" by not upgrading machines but it doesn't save money to have employees with tools that don't work well. Sometimes they just don't realize that computers don't last forever.
If you don't have a policy, here's a simple one you can start with: All computers are on a 3-year depreciation schedule. Every year the budget will include funds to replace 1/3rd of all machines. On the first day of each quarter enough machines will be ordered to replace the 9 percent oldest machines in the fleet.
CFOs like this because they like predictability. At one company the CFO was quite excited when I gave her control over which months the upgrades would happen. We agreed that 1/4 of the upgrades would happen each quarter; and she could pick which month that happened. She could even split it into individual monthly batches.
Instead of coming to the CFO to beg for new desktops now and then, it was a regular, scheduled activity. Less pain for everyone.
ProTip: At some companies servers are on a different depreciation schedule: they are designed to last longer and are on a 4-year depreciation schedule. On the other hand, their cost is amortized over all their users and therefore you can justify a 2-year schedule.
For More Information
See below links for more information on this topic: