I took This Old PC from Windows 98 to 2000. I won't go to XP. It came to me with Office 97. I won't upgrade it. I added free virus software Avast, which my friend Bruce says is better than Norton/Symantic. Firefox? Check. And I'm about to try IE 6. It's got Wi-Fi and Ethernet (both added by me). USB 1.1 (I haven't bothered to even think about 2.0).
Also added: Optical mouse (for no good reason); TDK CD burner (years ago) that never could burn an audio disc without clicks and dropouts; and Iomega ZIP USB (works flawlessly). I stuffed the box with as much memory as I could, and it recognizes 262 MB of it.
To change OS or Office versions at this point would be unwise -- it works as well as it's ever going to. If for some reason I need to open Word files in some "higher" version, I'll install Open Office, but I hope to avoid it.
This Old Mac, the Powerbook 14oo, went from OS 7.5.3 to 7.6.1. I added 32 MB of memory for a total of 48 MB. The best "new" software I got was LiteSwitch (from System 7 Today), which enables me to switch programs with Command-tab, just like in OS X and on Windows. And it is light.
The PB still runs Office 6. No plans to upgrade that, either. Even though that MS package is "Optimized for Power PC," it's slow enough. The best thing I did: Adding Internet Explorer 5 (which I also got from System 7 Today). I downloaded a bunch of stuff -- every version of Netscape I could find, iCab, Eudora and more, almost all of which I've deleted because it doesn't work (or work well). I kept WriteNow (but haven't used it much). What's nice is that 7.6.1 is loaded with great stuff, from valuable utilities to full-blown software (principally ClarisWorks).
It's all about hitting the sweet spot. Still, it's sometimes better to have an application work poorly (slowly) than not to have it at all. As long as you've got a swifter alternative (or a lot of patience).