I replace my entire PC every three years (Pentium III in 1999, Pentium 4 in 2002, Pentium D in 2005, and Core 2 Quad this year), and I upgrade software rather more often than that. Based on personal experience, that's how long a hard-worked system lasts before you get serious issues that can't be solved simply by reinstalling the OS and wiping off the smudges (and I know from several sources that Macs aren't much better). I'm surprised that businesses don't do that, too. Presumably, when you publish books, you budget for DTP software.