Hi there,
Please read carefully the following points:
1) If your phone is already slow on PR1.0, I would not consider upgrading to Belle. What you can do is to upgrade PR1.0 to Anna. Switching to Belle will make your phone even slower! But, finding Anna firmware may be very uneasy these days...
2) If you want to flash N8, you need Phoenix 2012.04 version (
https://app.box.com/s/a6w29u47wdyapq3tgqwq2bkgeg9us0zh). As you have neither a flashbox nor a dongle, you have to download the version which I call "Special package by Seidea". Highly likely it will be detected as a virus/malware, but this is not because there is real malware inside. No, it is because this version (as well as all other free Phoenix versions) was repacked and an activator was included in the EXE package, which is triggered by various antiviruses as a false positive.
3) Don't trust Chrome browser, it scolds that you have downloaded a virus every second time
There was no need to format your computers, of course.
4) If you do not trust Phoenix, you can grab Nokia CareSuite (
https://magentacloud.de/s/WJddkZ5WDrm9iow). This EXE file is directly from Nokia, and unlike Phoenix, original Nokia CareSuite does not require the flashbox.
I hope this piece of information helps.