How to measure productivity loss from slow PCs running Visual Studio? [closed]

Question

Many PCs we have on the development team are out-dated and are very slow to run Visual Studio 2008. They should very much be replaced with newer machines. But there's a general reluctance on management/company to buy new machines.

How do we come up with numbers and benchmarks to show that these slow PCs are causing a loss in productivity?

Obviously we can't call them to sit down with us as we build solutions and/or open various files.

Is there an objective way to come up with some kind of reliable numbers that non-technical people can understand?

It'd be nice to have a way to measure this across an entire organization on many different PCs running Visual Studio. I'm looking for an answer that does better than using a physical stopwatch. :)