To everyone who responded so generously to my question: thank you very
much!
I must admit I was surprised by the unanimous 'don't format and
reinstall' advice as I was under the impression this was only way to
know for sure you won't be stuck with left-over garbage from previous
installations / upgrades etc. on your harddrive. I love working on my
computer for about two months after I have reinstalled everything but
then it just seems to become slower and slower (and more unreliable)
every week that goes by.