@goldthreader
I frequently get "Not Responding" (in Windows title bar) in later (.8.x) versions.
XP SP3
P4 3.2
2G RAM
Calibre job priority
low
It is a
false message. Calibre is just too busy to respond right then.
Image conversion is
intensive (you might even hear the fans speed up). Let the conversion Run until Calibre errors out. 24 Hours

for some really nasty converts have been reported.
BTW the wheel pauses if you try to do other thing in calibre, that will slow the conversion down even more.
Your Anti-virus may also be scanning every file as calibre touches it.

Exclude the Library from the scan (your books were scanned when you added them from another location)