Would like to know the percentage of PCs outstanding that have Windows
XP SP2 installed.

I really need to estimate the percentage of PCs in Italy, of those that connect
to the internet through a dial up modem, have XP SP2. So if someone
can give me a rough estimate (with reference to the source of the
information) of the percentage of PCs in the US or in the US-EUROPE
countries in general that have XP SP2, I will
estimate the rest (assuming that PC that connect through a dial up
modem show a lower rate of conversion to XP SP2 due to the slower
download speed of the update, which should dissuade more people from
downloading it, although don't know how lower this rate actually is).

Try checking out the website OneStat.com. They seem to be a large
company that measures metrics of websites. Perhaps you can contact
them to find out the information you are looking for. Good luck!