What's the easiest and fastest way to compare 2 registry files?

"Run the installer and watch it with Sysinternals Process Monitor. You can filter the data so that only operations done by the installer are shown. You can even filter down to whatever operations you want to see (RegWrite, RegQueryValue, etc) and save the capture for later viewing.

Process monitor is freeware.

You could also use WinDiff. You can save exported registry files and then compare them afterwards:

WinDiff is free and open source.

Another FOSS solution for comparing actual registry exported files is RegShot.

RegShot is a small registry compare
utility that allows you to quickly
take a snapshot of your registry and
then compare it with a second one -
done after doing system changes or
installing a new software product. The
changes report can be produced in text
or HTML format and contains a list of
all modifications that have taken
place between snapshot1 and
snapshot2.In addition, you can also
specify folders (with sub filders) to
be scanned for changes as well."

"If you want to compare two registry files (that you have manually exported to text), then there are plenty of comparison apps, such as WinMerge, WinDiff, etc.

A word of caution: if the registry hive is large, then exporting can take a while, and worse, Windiff can more or less hang when trying to read large .reg files. Use WinMerge instead because it can handle large files, diff them faster, and the results are better (sometimes Windiff just gets it wrong).

If you want to compare the registry before and after an installation, then you may as well use an installation monitor. Again, there are plenty of them, but one of the main ones is PC Magazine’s InCtrl5 (you may have to pay to download it from their site, though the program itself is free, so there are plenty of sites that have it). They have released an update called InCtrlX which presumably, is better. Total Uninstall is a good commercial one. I generally like my old copy of InCtrl5, but Z-Soft Uninstaller has the same function and is exceptional (it’s the best of the dozen or so that I have tried); plus, it’s free.

Another solution is to analyse the installer itself. That is, look inside the EXE/MSI/BAT/INF/etc. file to see for yourself exactly what it does when it installs, including changes to files, registry entries, services, drivers, and such. That way you can avoid the whole detection process altogether. I find that 7Zip is the best way to look inside most executable installers (such as NSIS), because you can see the files and scripts and such. For MSI installers, I suggest either Microsoft’s own ORCA or InstEd. A good commercial tool for MSIs is AdvancedInstaller"